PHP - echo inside an echo - echo

Question

I have a PHP if/else statement. This is the code I'm trying to echo under an else condition.
<?php $locked = ForumData::is_topic_locked($post->topic_id);
if ($locked->topic_locked == 1) {echo '<td align="right"><font color="#FF0000">Topic Locked</font><td>';}
else {
echo '<td align="left"><img src="<?php echo SITE_URL?>/lib/skins/flyeuro/images/forums/t_reply.gif"/></td>'; }
?>
The bit I'm interested to echo is this.
<img src="<?php echo SITE_URL?>
If I try this... 'echo SITE_URL'
Parse error: syntax error, unexpected T_ECHO, expecting ',' or ';'
But this doesn't parse the image, and if I try parsing anything else, it's giving me parsing errors, which I can't fix?
How can I therefore produce an echo inside another echo?

Answer 1

why did you open a <?php tag again, you are already in echo line?
echo '<td align="left"><img src="'.SITE_URL.'/lib/skins/flyeuro/images/forums/t_reply.gif"/></td>';
and what is SITE_URL? Is that a variable, did you forget to put $?
echo prints out the string that you gave as parameter,
echo "foo";
As #hakre mentioned about it, . is used to concatenate strings.
$var = "foo"."bar"; //foobar
So you can use it in echo line,
$var = "foo"."bar"; //foobar
echo "foo "."bar ".$var // foo bar foobar
And It's not important weather variable defined as a string. It would be a constant variable.
define('SITE_URL', 'localhost:8080/phpvms');
echo "my website URL is ".SITE_URL; //my website URL is localhost:8080/phpvms

Answer 2

Remember:
<?php echo "View"; ?>
" and \
this!
Hope that's enough of a hint!#

Answer 3

Your problem is probably solved this way:
echo '<td align="left"><a href="',
url('Forum/create_new_post?topic_id=' . $post->topic_id . '&forum_id=' . $post->forum_id . '') ,
'"><img src="', SITE_URL,
#######################
'/lib/skins/flyeuro/images/forums/t_reply.gif"/></a></td>';
In PHP you can use constants quite like variables, e.g. to output them. You don't need to stack echoes inside each other or something.