PHP supports 'C', 'C++' and Unix shell-style (Perl style) comments. For example:
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 // ... ?> or # ... ?> WILL be printed: ?> breaks out of PHP mode and returns to HTML mode, and // or # cannot influence that. If the asp_tags configuration directive is enabled, it behaves the same with // %> and # %>. However, the </script> tag doesn't break out of PHP mode in a one-line comment.
'C' style comments end at the first */ encountered. Make sure you don't nest 'C' style comments. It is easy to make this mistake if you are trying to comment out a large block of code.