Possible Duplicate:
Commenting interpreted code and performance
Does comments affect when including files in PHP?
Let's say we have 100 class files and everytime when a page is requested, all these classes must be parsed by PHP.
Will PHP be slower if almost 1 half of the source code lines are the comments? Because usually I add a lot of comments & descriptions to code. This doesn't matter to compilers coz comments are not compiled, but PHP is interpreter, any bad thing may happen?
Yes, but it's minimal, and this can (and should) be addressed entirely by using APC or another opcode cache. As a bonus, APC will speed everything else up as well.
If your site is slow, comments are not the reason.