The following list gives an overview of which rights the PHP project reserves for itself, when choosing names for new internal identifiers. The definitive guide is the official CODING STANDARDS:
PHP owns the top-level namespace but tries to find decent descriptive names and avoid any obvious clashes.
Function names use underscores between words, while class names use the camel case rule (there are some exceptions for older classes and functions).
PHP will prefix any global symbols of an extension with the name of the extension. (In the past, there have been numerous exceptions to this rule.) Examples:
Iterators and Exceptions are however simply postfixed with "Iterator" and "Exception." Examples:
PHP reserves all symbols starting with __ as magical. It is recommended that you do not create symbols starting with __ in PHP unless you want to use documented magical functionality. Examples: