PHP comments
You can have comments in your PHP code, which serve no other purpose, but help the programmer better understand what a specific part of the code is about. Comments are useful and important regardless if you're the only programmer working on a piece of code. A simple rule of thumb is - the more time a part of the code takes to write, the more it needs comments.
One-line comments
In any part of the script anything after // or # is ignored by PHP (except if it's in a string within quotes). For example:
// spitting out some date/time
echo 'Today is ', date('Y-m-d'); // printing the current date
echo ' and it is now ', date('H:i:s'); # the time
The result of this is
Today is 2007-09-06 and it is now 23:00:47
As you can see, everything after // and # was ignored.
In practice
In practice, # comments (Unix shell style comments) are very rarely used, most people use // (the C-style comments). So you're probably better off using only // for one-line comments.
Multi-line comments
You can put comments on multiple lines if you surround them in /* and */. An example:
/*
This code prints
date and time
*/
echo 'Today is ', date('Y-m-d'); // printing the current date
echo ' and it is now ', date('H:i:s'); # the time
Documentation from comments
The tool called phpDoc can take comments written in a certain format and create documentation from them. This is pretty cool (you write documentation without actually writing it) and any decent size project should benefit from this tool.
Leaving PHP in a comment
If you have a PHP ending tag inside a single line comment, you'll leave PHP. Inside a multi-line comment the end tag is considered part of the comment and doesn't have the meaning of ending the PHP block. Example:
<?php /* PHP code starts with <?php and end with ?> */ // ?> ends a section of PHP code ?>
The result of this will be
ends a section of PHP code ?>
That's because the ?> in the single-line comment ended the script section, and everything after that is just normal plain HTML