First of all php is an APACHE module, so it could be confusing...
PHP can be installed as a module on APACHE (or any other web server) or
as CGI and work like an external binary executable application (php.exe on windows as wizard said).
A php module can be defined as an extra library or application added on PHP to give additional functions to it. Modules can either be compiled with PHP (like GD on 4.3.x) or can be loaded when php starts, through php.ini configuration (like imap.so).