Checkmark the box, then click Save to restart Apache with PHP enabled
Click on Web in the service list
Then Settings
Modules
Put a checkmark in the php5_module box
Click Save which will restart Apache to effect the change. A little rotating gear will appear, then disappear to signal that Apache has re-started successfully. There will be no problems if simply enabling the default shipped version of PHP.
In cases of customized php versions or extensions that fail to load, the "Stop Web" button will change to "Start Web" and the Web service ball will turn from green to gray or orange to signal a problem starting Apache.
Custom php versions and extensions work much the same on OS X as on any linux-unix. See appendix below or go to php.net for more info.