Single comments causing errors in PHP

This is actually rather subtle, but can be seen more clearly with a simpler example:

<?php echo 'hello'; // This is a comment with '?>' in ?> 

Which results in this:

hello' in ?> 

At a glance, it looks like the first ?> is part of the comment, and the <?php block will continue until the second one (and at the time I write this, the site's syntax highlighter assumes so); but PHP doesn't parse it that way.

To see why, consider this example:

<?php echo 'hello'; // this is a comment ?> 

This is a self-contained piece of PHP which you could embed in a template file, with a comment contained inside the <?php ... ?> block. If the comment suppressed the meaning of ?>, this wouldn't work.

So rather than "comment until the end of the line", // is defined as "comment until the end of the line or the end of the current PHP block, whichever comes first".

In contrast, a block comment is defined as ending at its own */ marker, so the following does comment out the first ?>:

<?php echo 'hello'; /* This is a comment with '?>' in */ ?> 

Output:

hello 

In your case, you effectively have this (with some other code before and after):

<?php //$conText = preg_replace_callback('@(<a .*?href\s*=\s*(([\'"])(.*?)\3|([^\s>]+)).*?>(.*?)</a>)@si', create_function('$matches', 'return addLink($matches, 1);'), $conText); ?> 

Buried in the commented out line happens to be the characters ?>. The parser doesn't look at the rest of the syntax, because it's a comment, but does pick up those characters as ending the PHP block.

The result is that PHP interprets it essentially as this:

<?php //$conText = preg_replace_callback('@(<a .*?href\s*=\s*(([\'"])(.*?)\3|([^\s>]+)).* ?> (.*?)</a>)@si', create_function('$matches', 'return addLink($matches, 1);'), $conText); ?> 

That leaves the end of the line which you were trying to comment out, and everything after it, as text to be output rather than code.