PHP comments: # vs. //

It doesn't change a single thing, I'd say ; it's just a matter of habits :

• // comes from C, and exists in several languages used by lots of people.
• # comes from shell and Perl, and exists in not as many "important" languages as // -- so less people use it.

And why are those two both available in PHP ? Well, PHP has been built with people knowing both C, Shell, and Perl, and they have brought what they liked from those languages ;-)

I'd probably stick with // simply because it's the same in JavaScript, and when you code PHP, you're likely going to have some JavaScript in your files as well. (Most websites use JavaScript nowadays.)

This way, your comments look the same, regardless of the language, and it'd ease readability a small bit.

For single line comments, there is no significant technical reason for prefering // over the octothorpe (that's what the Zend PHP 5 Certification Study Guide calls #) or vice versa. // seems to the more common choice out there in the wild.

I would only suggest you use // or /* */. As your programs grow you may wish to use a documentator like phpdoc to automate documentation for your program and most of these documentators as well as IDE's will only accept these two.