Google App Engine version 1.8.5 clearly says that "the Mac OSX SDK now includes a PHP interpreter, installing PHP separately is no longer required." However, I don't seem to find it. I know I could just install php-cgi on my own, but if it were indeed included in the SDK, I wouldn't want to install another copy.
It is here: /Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/php-cgi