You have two ways to enable it.
First, you can set the absolute path of the php module file in your httpd.conf file like this:
LoadModule php5_module /path/to/mods-available/libphp5.so
Second, you can link the module file to the mods-enabled directory:
ln -s /path/to/mods-available/libphp5.so /path/to/mods-enabled/libphp5.so