If you're using OSX you can open Terminal by typing:
cmd + space bar
or just going to Applications -> Utilities and clicking on the Terminal icon.
On the command line in Terminal type:
The -v flag means version. This will show you which version of PHP you are using. For more options to the php command you can type man php.
Sometimes, when you are busy or out and about and your iPhone starts ringing, you don't want to pick it up, but you know it might be so-and-so with that important call you've been waiting on.