Has anyone documented the differences between Stack Exchange Markup and Github Markup?
I'm in the midst of a project to convert Stack Exchange Markdown to Github Markdown. It might be a little more complicated because Jekyll on Github Pages uses a Markdown derivative called "Kramdown".
I've already written some of the conversion in my Python program. For example old SE posts with #Header must be converted to # Header.
Another example are "> Block quote" lines have two spaces appended to the end of the line.
Now it's starting to get tricky (for me at least) where in an image in SE is specified as:
[![Ubuntu 5 DE.png][1]][1]
**Note:** Blah, blah, blah
[1]: https://i.stack.imgur.com/MoxHd.jpg
It has to be converted to Github image markdown format:
**Note:** Blah, blah, blah
Another example of "footer hyper links" (for lack of a better noun) in Stack Exchange Markdown is:
- [Jack Master Volume?][1]
The simplest solution then is to install [JackMix][2]:
find listed [here][3].
[this script][4] is where you are heading:
[1]: https://discourse.ardour.org/t/jack-master-volume/84650
[2]: http://www.arnoldarts.de/jackmix/.
[3]: http://jackaudio.org/applications/
[4]: https://unix.stackexchange.com/questions/374085/lower-or-increase-pulseaudio-volume-on-all-outputs
Needs to be converted to Github Markdown format of:
- [Jack Master Volume?](https://discourse.ardour.org/t/jack-master-volume/84650)
The simplest solution then is to install [JackMix](http://www.arnoldarts.de/jackmix/.):
find listed [here](http://jackaudio.org/applications/).
[this script](https://unix.stackexchange.com/questions/374085/lower-or-increase-pulseaudio-volume-on-all-outputs) is where you are heading:
Finally tonight I discovered that in Stack Exchange you can have:
<!-- language: bash -->
#!/bin/bash
cat "$Filename.zip" | base64 > "$Filename64"
That needs reformatting to Github Markdown like this:
``` bash
#!/bin/bash
cat "$Filename.zip" | base64 > "$Filename64"
```
It gets even more complicated when SE Markdown has:
<!-- language-all: lang-bash -->
Or it has this:
<pre><code>Some lines of code
some more lines of
code </code></pre>
An existing Github Repo to convert would be awesome! If not then if someone has documented the differences between Stack Exchange Markup and Github Markup that would be great too.
If this question goes unanswered for a month then I guess I'll be answering it eventually after the trial-error-fix process is finished.
The self-answer seems to answer a different question, so I'll answer the one posed above.
Has anyone documented the differences between Stack Exchange Markup and Github Markup?
As of mid-2020, Stack Exchange uses CommonMark with support for a few custom features "like spoilers, MathJax, circuit diagrams, stack snippets, etc."
GitHub uses their own dialect, GitHub Flavored Markdown (GFM). The most notable extensions that GFM introduces are probably tables (supported on SE since late 2020), fenced code blocks (supported on SE since early 2019), and task lists (unsupported but also not really necessary on SE).
Most of the examples shown above work fine when rendered on Stack Exchange or by a compliant GFM implementation, but let's look at them in turn:
For example old SE posts with #Header must be converted to # Header.
This should have been cleaned up by the migration to CommonMark:
we will run a big migration across the network that will convert existing posts to use the new CommonMark format
Another example are "> Block quote" lines have two spaces appended to the end of the line.
Two spaces at the end of the line indicate a line break in Markdown, going all the way back to the original implementation.
Blockquotes do not require such line breaks, though they will rewrap without them. Line breaks and blockquotes are unrelated features.
Your image examples are interchangeable in both formats. Let's look at the SE one:
[![Ubuntu 5 DE.png][1]][1]
**Note:** Blah, blah, blah
[1]: https://i.stack.imgur.com/MoxHd.jpg
There is nothing specific to Stack Exchange here.
The [foo][1]… [1]: https://... syntax is a reference-style link which, again, comes from the original project. It is equivalent to the inline form [foo](https://...). Both forms work on both platforms. ![Ubuntu 5 DE.png][1]… [1]: https://i.stack.imgur.com/MoxHd.jpg is the same as .
On SE images are wrapped by a link by default, which introduces the wrapping [...][1]. But, again, this is not SE-specific. It's like comparing <img src="..."> to <img src="...">. This also works on both platforms.
Another example of "footer hyper links" (for lack of a better noun)
These are reference-style links already discussed and present in every version of Markdown I've ever seen, including the original project. They work in GFM as well as they do in CommonMark. No conversion is necessary.
Finally tonight I discovered that in Stack Exchange you can have:
<!-- language: bash -->
#!/bin/bash
cat "$Filename.zip" | base64 > "$Filename64"
That needs reformatting to Github Markdown like this:
``` bash
#!/bin/bash cat "$Filename.zip" | base64 > "$Filename64"
```
This is the only example above that requires any special behaviour at all. However, it mostly works out of the box. GFM supports indented code blocks and SE has supported fenced code blocks for over three years.
GFM doesn't understand legacy HTML-style SE language hints <!-- language: bash --> and <!-- language-all: ... -->, so it will render such code blocks without syntax highlighting. But they will still render as a code block.
The last example is just embedded HTML that both platforms (and the original) know how to render:
<pre><code>Some lines of code
some more lines of
code </code></pre>
With the exception of HTML-style language hints, every example you show above works out of the box on both Stack Exchange and GitHub Flavored Markdown. No conversion necessary.
Converting thousands of Stack Exchange Q&A in markdown format isn't as easy
as simply copying them over to GitHub Pages. The python program
stack-to-blog.py was used to convert Stack Exchange posts to
GitHub Pages Posts.
The full stack-to-blog.py program can be accessed on the
Pippim Website repo 🔗.
The program automatically:
Creates Jekyll front matter on posts and front matter totals for site.
Selects Stack Exchange Posts based on meeting minimum criteria such as up-votes or accepted answer status.
If self-answered question, the answer is included and not the question.
If self-answered question, the accepted answer alone doesn't qualify. Votes from other are the qualifier.
Initial testing allows selecting small set of random record numbers to convert.
Converts Stack Exchange Markdown formats to GitHub Pages Kramdown Markdown format.
Creates hyperlinks to original Answer in Stack Exchange and Kramdown in GitHub Pages.
Creates search word to URL indices excluding 50% of words like "a", "the", etc. to save space.
Selectively inserts Table of Contents based on minimum criteria settings.
Selectively inserts Section Navigation Buttons for: Top (Top of Page), ToS (Top of Section), ToC (Table of Contents) and Skip (Skip section).
Selectively inserts "Copy Code Block to System Clipboard" button based on lines of code.
Creates HTML with "Top Ten Answers" with the most votes.
Creates powerful nested expandable/collapsible detail/summary HTML for many thousands of tags by post.
Remaps hyperlinks in Stack Exchange Posts to {{ site.title }} website posts if they were converted.
Fixes old broken #header Stack Exchange Markdown.
Converts < block quote Stack Exchange Markdown into what works in Jekyll Kramdown.
Convert Stack Exchange <!-- language --> tags to fenced code block language.
When no fenced code block language is provided, uses shebang language first (if available).
Converts older four-space indented code blocks to fenced code blocks.
Converts Stack Exchange Hyperlinks where the website post title is implied and not explicit.
Prints list of self-answered questions that were not accepted after the mandatory two day wait period.
Prints list of Rouge Syntax Highlighting languages not supported in fenced code blocks.
Prints summary totals when finished.
Full documentation is provided here.
This is what program looks like when running:
Whenever I have to edit Perl Mason files, I always have problems indenting lines due to Perl code starting with %. For example:
<div>
<div>
% if( !$something ) {
<strong><% $title %></strong>
% }
</div>
</div>
Any idea how I can tell Vim to ignore the % at the beginning of the line and indent like it wasn't there?
I'm using https://github.com/aming/vim-mason to support the mixed Perl/HTLM syntax, but I don't think it changes anything.
This is Perl code embedded inside HTML, so the indenting comes from $VIMRUNTIME/indent/html.vim. This defines an 'indentexpr', implemented by HtmlIndent().
You need to modify that implementation to ignore % in the first column; whenever it accesses the buffer (getline(), prevnonblank(), shiftwidth()), you need to intercept, find the previous line that does have such % sigil, and return the value for that instead. (If those special lines can also contain HTML tags, you may have to extract those from the Perl code and return only those.) That gets you the indenting you desire.
Unfortunately, it's not trivial, and you have to fork the original implementation. However, if you manage to implement a clean solution, you could suggest adding integration points to the author of indent/html.vim. If there are other languages apart from Mason that use these prefixes on top of HTML, that would be an additional argument for adding such support (and maybe even your wrapper functions).
It seems that Github does not support the sphinx.ext.doctest syntax in their reStructuredText renderer, which is causing some problems when trying to include a doctest-style code block (.. doctest) in a README.rst which is transcluded into the documentation index (which is rendered by sphinx). If I replace the .. doctest directive with something else, it doesn't render properly as a doctest on Sphinx, but if I don't remove the directive, the code block doesn't render at all (see this gist).
Ideally I'd like to find a solution which just does the right thing in both environments, but failing that, is there a way to fall back to a .. code block or some other supported format (e.g. the rST equivalent of a <NoScript> tag)?
I use Pod::Weaver with Dist::Zilla. It does several good things for me. It adds POD section VERSION, AUTHOR, LICENSE automatically, and in my source code I can use simple POD syntax, I can write "=method new" and it will be converted to correct POD.
Now I wanted to add an image to the POD. To do it I need to add some HTML. So I'm writing in my source code:
=begin HTML
<p>
<a href="http://upload.bessarabov.ru/bessarabov/031VBX4pHw_ALPcxRTVjflnAWuc.png">
<img src="http://upload.bessarabov.ru/bessarabov/VdagpUXEQdMslOqUyOAzwa-DOaU.png" width="500" height="125" alt="Status board graph sample" />
</a>
</p>
=end HTML
Then I write dzil release and release the module on CPAN. After uploading to CPAN I recognize that my HTML POD was changed by Pod::Weaver and now it looks like:
=for HTML <p>
<p>
<a href="http://upload.bessarabov.ru/bessarabov/031VBX4pHw_ALPcxRTVjflnAWuc.png">
<img src="http://upload.bessarabov.ru/bessarabov/VdagpUXEQdMslOqUyOAzwa-DOaU.png" width="500" height="125" alt="Status board graph sample" />
</a>
</p>
And this HTML part has been moved in the POD. I wanted it to be just after SYNOPSIS part, but not it is after the last method.
I still want to use Pod::Weaver, because it does a lot of good things, but I want HTML to be put in the exact place of the POD and not to be converted.
How can I do it?
After much reading of perldoc perlpod and testing, the following things are apparent:
According to spec, =begin html segments can be translated safely to =for html segments.
However, this may only occur when the body of the segment contains segments that are grouped in paragraphs. So foo\nbar can be translated, but foo\n\nbar cannot.
The module that is doing the transformation from =begin to =for, when seeing a double \n in its body, refuses to emit =for, and instead, reverts to emitting =begin.
Given what number 3 Here is, If you want to force it to emit =begin html, simply spicing it up with a few extra \n's will do the trick.
However, the problem remains, that for whatever reason, what appears to be valid POD format given, is not rendering as intended.
Either
A. The Pod2HTML layer is not coded right to respond to =for ( in which case, a few extra \n's to force an =begin may help.
B. There's a security level thats preventing you from using complex HTML ( which may entail hyperlinks in images, which could be an XSS security threat )
If the problem is B, then you're not going to get around that by being tricky.
Though I assure you, displaying images IS doable, I do it myself
This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
When running my script, I am getting several errors like this:
Warning: Cannot modify header information - headers already sent by (output started at /some/file.php:12) in /some/file.php on line 23
The lines mentioned in the error messages contain header() and setcookie() calls.
What could be the reason for this? And how to fix it?
No output before sending headers!
Functions that send/modify HTTP headers must be invoked before any output is made.
summary ⇊
Otherwise the call fails:
Warning: Cannot modify header information - headers already sent (output started at script:line)
Some functions modifying the HTTP header are:
header / header_remove
session_start / session_regenerate_id
setcookie / setrawcookie
Output can be:
Unintentional:
Whitespace before <?php or after ?>
The UTF-8 Byte Order Mark specifically
Previous error messages or notices
Intentional:
print, echo and other functions producing output
Raw <html> sections prior <?php code.
Why does it happen?
To understand why headers must be sent before output it's necessary
to look at a typical HTTP
response. PHP scripts mainly generate HTML content, but also pass a
set of HTTP/CGI headers to the webserver:
HTTP/1.1 200 OK
Powered-By: PHP/5.3.7
Vary: Accept-Encoding
Content-Type: text/html; charset=utf-8
<html><head><title>PHP page output page</title></head>
<body><h1>Content</h1> <p>Some more output follows...</p>
and <img src=internal-icon-delayed>
The page/output always follows the headers. PHP has to pass the
headers to the webserver first. It can only do that once.
After the double linebreak it can nevermore amend them.
When PHP receives the first output (print, echo, <html>) it will
flush all collected headers. Afterward it can send all the output
it wants. But sending further HTTP headers is impossible then.
How can you find out where the premature output occurred?
The header() warning contains all relevant information to
locate the problem cause:
Warning: Cannot modify header information - headers already sent by
(output started at /www/usr2345/htdocs/auth.php:52) in
/www/usr2345/htdocs/index.php on line 100
Here "line 100" refers to the script where the header() invocation failed.
The "output started at" note within the parenthesis is more significant.
It denominates the source of previous output. In this example, it's auth.php
and line 52. That's where you had to look for premature output.
Typical causes:
Print, echo
Intentional output from print and echo statements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that. Use functions
and templating schemes. Ensure header() calls occur before messages
are written out.
Functions that produce output include
print, echo, printf, vprintf
trigger_error, ob_flush, ob_end_flush, var_dump, print_r
readfile, passthru, flush, imagepng, imagejpeg
among others and user-defined functions.
Raw HTML areas
Unparsed HTML sections in a .php file are direct output as well.
Script conditions that will trigger a header() call must be noted
before any raw <html> blocks.
<!DOCTYPE html>
<?php
// Too late for headers already.
Use a templating scheme to separate processing from output logic.
Place form processing code atop scripts.
Use temporary string variables to defer messages.
The actual output logic and intermixed HTML output should follow last.
Whitespace before <?php for "script.php line 1" warnings
If the warning refers to output inline 1, then it's mostly
leading whitespace, text or HTML before the opening <?php token.
<?php
# There's a SINGLE space/newline before <? - Which already seals it.
Similarly it can occur for appended scripts or script sections:
?>
<?php
PHP actually eats up a single linebreak after close tags. But it won't
compensate multiple newlines or tabs or spaces shifted into such gaps.
UTF-8 BOM
Linebreaks and spaces alone can be a problem. But there are also "invisible"
character sequences that can cause this. Most famously the
UTF-8 BOM (Byte-Order-Mark)
which isn't displayed by most text editors. It's the byte sequence EF BB BF, which is optional and redundant for UTF-8 encoded documents. PHP however has to treat it as raw output. It may show up as the characters  in the output (if the client interprets the document as Latin-1) or similar "garbage".
In particular graphical editors and Java-based IDEs are oblivious to its
presence. They don't visualize it (obliged by the Unicode standard).
Most programmer and console editors however do:
There it's easy to recognize the problem early on. Other editors may identify
its presence in a file/settings menu (Notepad++ on Windows can identify and
remedy the problem),
Another option to inspect the BOMs presence is resorting to an hexeditor.
On *nix systems hexdump is usually available,
if not a graphical variant which simplifies auditing these and other issues:
An easy fix is to set the text editor to save files as "UTF-8 (no BOM)"
or similar to such nomenclature. Often newcomers otherwise resort to creating new files and just copy&pasting the previous code back in.
Correction utilities
There are also automated tools to examine and rewrite text files
(sed/awk or recode).
For PHP specifically there's the phptags tag tidier.
It rewrites close and open tags into long and short forms, but also easily
fixes leading and trailing whitespace, Unicode and UTF-x BOM issues:
phptags --whitespace *.php
It's safe to use on a whole include or project directory.
Whitespace after ?>
If the error source is mentioned as behind the
closing ?>
then this is where some whitespace or the raw text got written out.
The PHP end marker does not terminate script execution at this point. Any text/space characters after it will be written out as page content
still.
It's commonly advised, in particular to newcomers, that trailing ?> PHP
close tags should be omitted. This eschews a small portion of these cases.
(Quite commonly include()d scripts are the culprit.)
Error source mentioned as "Unknown on line 0"
It's typically a PHP extension or php.ini setting if no error source
is concretized.
It's occasionally the gzip stream encoding setting
or the ob_gzhandler.
But it could also be any doubly loaded extension= module
generating an implicit PHP startup/warning message.
Preceding error messages
If another PHP statement or expression causes a warning message or
notice being printed out, that also counts as premature output.
In this case you need to eschew the error,
delay the statement execution, or suppress the message with e.g.
isset() or #() -
when either doesn't obstruct debugging later on.
No error message
If you have error_reporting or display_errors disabled per php.ini,
then no warning will show up. But ignoring errors won't make the problem go
away. Headers still can't be sent after premature output.
So when header("Location: ...") redirects silently fail it's very
advisable to probe for warnings. Reenable them with two simple commands
atop the invocation script:
error_reporting(E_ALL);
ini_set("display_errors", 1);
Or set_error_handler("var_dump"); if all else fails.
Speaking of redirect headers, you should often use an idiom like
this for final code paths:
exit(header("Location: /finished.html"));
Preferably even a utility function, which prints a user message
in case of header() failures.
Output buffering as a workaround
PHPs output buffering
is a workaround to alleviate this issue. It often works reliably, but shouldn't
substitute for proper application structuring and separating output from control
logic. Its actual purpose is minimizing chunked transfers to the webserver.
The output_buffering=
setting nevertheless can help.
Configure it in the php.ini
or via .htaccess
or even .user.ini on
modern FPM/FastCGI setups.
Enabling it will allow PHP to buffer output instead of passing it to the webserver instantly. PHP thus can aggregate HTTP headers.
It can likewise be engaged with a call to ob_start();
atop the invocation script. Which however is less reliable for multiple reasons:
Even if <?php ob_start(); ?> starts the first script, whitespace or a
BOM might get shuffled before, rendering it ineffective.
It can conceal whitespace for HTML output. But as soon as the application logic attempts to send binary content (a generated image for example),
the buffered extraneous output becomes a problem. (Necessitating ob_clean()
as a further workaround.)
The buffer is limited in size, and can easily overrun when left to defaults.
And that's not a rare occurrence either, difficult to track down
when it happens.
Both approaches therefore may become unreliable - in particular when switching between
development setups and/or production servers. This is why output buffering is
widely considered just a crutch / strictly a workaround.
See also the basic usage example
in the manual, and for more pros and cons:
What is output buffering?
Why use output buffering in PHP?
Is using output buffering considered a bad practice?
Use case for output buffering as the correct solution to "headers already sent"
But it worked on the other server!?
If you didn't get the headers warning before, then the output buffering
php.ini setting
has changed. It's likely unconfigured on the current/new server.
Checking with headers_sent()
You can always use headers_sent() to probe if
it's still possible to... send headers. Which is useful to conditionally print
info or apply other fallback logic.
if (headers_sent()) {
die("Redirect failed. Please click on this link: <a href=...>");
}
else{
exit(header("Location: /user.php"));
}
Useful fallback workarounds are:
HTML <meta> tag
If your application is structurally hard to fix, then an easy (but
somewhat unprofessional) way to allow redirects is injecting a HTML
<meta> tag. A redirect can be achieved with:
<meta http-equiv="Location" content="http://example.com/">
Or with a short delay:
<meta http-equiv="Refresh" content="2; url=../target.html">
This leads to non-valid HTML when utilized past the <head> section.
Most browsers still accept it.
JavaScript redirect
As alternative a JavaScript redirect
can be used for page redirects:
<script> location.replace("target.html"); </script>
While this is often more HTML compliant than the <meta> workaround,
it incurs a reliance on JavaScript-capable clients.
Both approaches however make acceptable fallbacks when genuine HTTP header()
calls fail. Ideally you'd always combine this with a user-friendly message and
clickable link as last resort. (Which for instance is what the http_redirect()
PECL extension does.)
Why setcookie() and session_start() are also affected
Both setcookie() and session_start() need to send a Set-Cookie: HTTP header.
The same conditions therefore apply, and similar error messages will be generated
for premature output situations.
(Of course, they're furthermore affected by disabled cookies in the browser
or even proxy issues. The session functionality obviously also depends on free
disk space and other php.ini settings, etc.)
Further links
Google provides a lengthy list of similar discussions.
And of course many specific cases have been covered on Stack Overflow as well.
The WordPress FAQ explains How do I solve the Headers already sent warning problem? in a generic manner.
Adobe Community: PHP development: why redirects don't work (headers already sent)
Nucleus FAQ: What does "page headers already sent" mean?
One of the more thorough explanations is HTTP Headers and the PHP header() Function - A tutorial by NicholasSolutions (Internet Archive link).
It covers HTTP in detail and gives a few guidelines for rewriting scripts.
This error message gets triggered when anything is sent before you send HTTP headers (with setcookie or header). Common reasons for outputting something before the HTTP headers are:
Accidental whitespace, often at the beginning or end of files, like this:
<?php
// Note the space before "<?php"
?>
To avoid this, simply leave out the closing ?> - it's not required anyways.
Byte order marks at the beginning of a php file. Examine your php files with a hex editor to find out whether that's the case. They should start with the bytes 3F 3C. You can safely remove the BOM EF BB BF from the start of files.
Explicit output, such as calls to echo, printf, readfile, passthru, code before <? etc.
A warning outputted by php, if the display_errors php.ini property is set. Instead of crashing on a programmer mistake, php silently fixes the error and emits a warning. While you can modify the display_errors or error_reporting configurations, you should rather fix the problem.
Common reasons are accesses to undefined elements of an array (such as $_POST['input'] without using empty or isset to test whether the input is set), or using an undefined constant instead of a string literal (as in $_POST[input], note the missing quotes).
Turning on output buffering should make the problem go away; all output after the call to ob_start is buffered in memory until you release the buffer, e.g. with ob_end_flush.
However, while output buffering avoids the issues, you should really determine why your application outputs an HTTP body before the HTTP header. That'd be like taking a phone call and discussing your day and the weather before telling the caller that he's got the wrong number.
I got this error many times before, and I am certain all PHP programmer got this error at least once before.
Possible Solution 1
This error may have been caused by the blank spaces before the start of the file or after the end of the file.These blank spaces should not be here.
ex)
THERE SHOULD BE NO BLANK SPACES HERE
echo "your code here";
?>
THERE SHOULD BE NO BLANK SPACES HERE
Check all files associated with file that causes this error.
Note: Sometimes EDITOR(IDE) like gedit (a default linux editor) add one blank line on save file. This should not happen. If you are using Linux. you can use VI editor to remove space/lines after ?> at the end of the page.
Possible Solution 2:
If this is not your case, then use ob_start to output buffering:
<?php
ob_start();
// code
ob_end_flush();
?>
This will turn output buffering on and your headers will be created after the page is buffered.
Instead of the below line
//header("Location:".ADMIN_URL."/index.php");
write
echo("<script>location.href = '".ADMIN_URL."/index.php?msg=$msg';</script>");
or
?><script><?php echo("location.href = '".ADMIN_URL."/index.php?msg=$msg';");?></script><?php
It'll definitely solve your problem.
I faced the same problem but I solved through writing header location in the above way.
You do
printf ("Hi %s,</br />", $name);
before setting the cookies, which isn't allowed. You can't send any output before the headers, not even a blank line.
COMMON PROBLEMS:
(copied from: source)
====================
1) there should not be any output (i.e. echo.. or HTML codes) before the header(.......); command.
2) remove any white-space(or newline) before <?php and after ?> tags.
3) GOLDEN RULE! - check if that php file (and also, if you include other files) have UTF8 without BOM encoding (and not just UTF-8). That is problem in many cases (because UTF8 encoded file has something special character in the start of php file, which your text-editor doesnt show)!!!!!!!!!!!
4) After header(...); you must use exit;
5) always use 301 or 302 reference:
header("location: http://example.com", true, 301 ); exit;
6) Turn on error reporting, and find the error. Your error may be caused by a function that is not working. When you turn on error reporting, you should always fix top-most error first. For example, it might be "Warning: date_default_timezone_get(): It is not safe to rely on the system's timezone settings." - then farther on down you may see "headers not sent" error. After fixing top-most (1st) error, re-load your page. If you still have errors, then again fix the top-most error.
7) If none of above helps, use JAVSCRIPT redirection(however, strongly non-recommended method), may be the last chance in custom cases...:
echo "<script type='text/javascript'>window.top.location='http://website.com/';</script>"; exit;
It is because of this line:
printf ("Hi %s,</br />", $name);
You should not print/echo anything before sending the headers.
A simple tip: A simple space (or invisible special char) in your script, right before the very first <?php tag, can cause this !
Especially when you are working in a team and somebody is using a "weak" IDE or has messed around in the files with strange text editors.
I have seen these things ;)
Another bad practice can invoke this problem which is not stated yet.
See this code snippet:
<?php
include('a_important_file.php'); //really really really bad practise
header("Location:A location");
?>
Things are okay,right?
What if "a_important_file.php" is this:
<?php
//some php code
//another line of php code
//no line above is generating any output
?>
----------This is the end of the an_important_file-------------------
This will not work? Why?Because already a new line is generated.
Now,though this is not a common scenario what if you are using a MVC framework which loads a lots of file before handover things to your controller? This is not an uncommon scenario. Be prepare for this.
From PSR-2 2.2 :
All PHP files MUST use the Unix LF (linefeed) line ending.
All PHP files MUST end with a single blank line.
The closing ?> tag MUST be omitted from files containing only php
Believe me , following thse standards can save you a hell lot of hours from your life :)
Sometimes when the dev process has both WIN work stations and LINUX systems (hosting) and in the code you do not see any output before the related line, it could be the formatting of the file and the lack of Unix LF (linefeed)
line ending.
What we usually do in order to quickly fix this, is rename the file and on the LINUX system create a new file instead of the renamed one, and then copy the content into that. Many times this solve the issue as some of the files that were created in WIN once moved to the hosting cause this issue.
This fix is an easy fix for sites we manage by FTP and sometimes can save our new team members some time.
Generally this error arise when we send header after echoing or printing. If this error arise on a specific page then make sure that page is not echoing anything before calling to start_session().
Example of Unpredictable Error:
<?php //a white-space before <?php also send for output and arise error
session_start();
session_regenerate_id();
//your page content
One more example:
<?php
includes 'functions.php';
?> <!-- This new line will also arise error -->
<?php
session_start();
session_regenerate_id();
//your page content
Conclusion: Do not output any character before calling session_start() or header() functions not even a white-space or new-line