What best describes a PHP extension?
A: A class that extends from one of the standard PHP library (SPL) classes.
B: A class with an common interface so that it can be interchanged.
C: An add-on or plug-in that adds (or changes) core functionality to PHP.
D: A PHP library, but only if it uses namespaces.
E: That is the extension that the Web server uses to identify php files when you have to run several versions of PHP on the same server. For instance this way you can register "php" and "php4" as PHP extensions to select the correct scripting engine.