Find the version of OSX the user is running with a bash command: 

sw_vers -productVersion

It outputs the version number, so you'll get something like "10.9" or "10.8.4" which you can then parse further for however you need.

Use it in Applescript:

set the_version to do shell script "sw_vers -productVersion"

Use it in PHP

$version = exec('sw_vers -productVersion');