PHP Pocket Reference, 2nd Edition (2009)
Variable names in PHP are case-sensitive. That means $A and $a are two distinct variables. However, function names in PHP are not case-sensitive. This rule applies to both built-in functions and user-defined functions.
PHP ignores whitespace between tokens. You can use spaces, tabs, and newlines to format and indent your code to make it more readable. PHP statements are terminated by semicolons.
There are three types of comments in PHP:
/* C style comments */
// C++ style comments
# Bourne shell style comments
The C++ and Bourne shell-style comments can be inserted anywhere in your code. Everything from the comment characters to the end of the line is ignored. The C-style comment tells PHP to ignore everything from the start of the comment until the end-comment characters. This means that this style of comment can span multiple lines.