Rewrite last part of file read - fwrite

Question

I see how to change the pointer in a file to read just part of the file (fseek).
Does setting the pointer for rewriting ('fwrite') part of the file work the same way?
After the 'fread' do I leave pointer alone or backup from current position to rewrite what I just read?
Note the idea is to exchange two blocks of the file for randomization.

Answer 1

Yes fwrite works like fread. After fread, you need to fseek to the position to write to. Here's an example:
// get temporary directory
$tmp_dir = sys_get_temp_dir() . DIRECTORY_SEPARATOR;
// get temporary file name in temporary dir
$tmp_filename = tempnam($tmp_dir, '');
// open temp file for read and write
$handle = fopen($tmp_filename, 'r+');
$str = 'the quick brown fox jumps over the lazy dog';
echo($str . PHP_EOL);
// write string to file
$result = fwrite($handle, $str);
$pos = strpos($str, 'jumps');
// seek to jumps position
$result = fseek($handle, $pos);
// change jumps for sneaks under
$str2 = 'sneaks under';
$result = fwrite($handle, $str2);
// get file size
$file_size = filesize($tmp_filename);
// seek to file start
$result = fseek($handle, 0);
// read from file
$result = fread($handle, $file_size);
echo($result);
// close file
fclose($handle);
$ php src/fwrite.php
the quick brown fox jumps over the lazy dog
the quick brown fox sneaks under the lazy dog