PHP supports 'C', 'C++' and Unix shell-style (Perl style) comments. For example:
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<?php | 
The "one-line" comment styles only comment to the end of the line or the current block of PHP code, whichever comes first. This means that HTML code after // ?> WILL be printed: ?> skips out of the PHP mode and returns to HTML mode, and // cannot influence that. If asp_tags configuration directive is enabled, it behaves the same with // %>. However, </script> tag doesn't escape PHP mode in one-line comment.
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<h1>This is an <?php # echo 'simple';?> example.</h1> | 
'C' style comments end by the first encountered */. You should be careful not to nest 'C' style comments, which can happen when commenting out large blocks.