It is very unlikely that you will get usable code out of a PHP decompiler - it is an interpreted language, so the results will be patchy at best (and PHP is not a good language for readability in the first place so you are onto a loser right from the start.)
Bear in mind that if this is not your code you could well be infringing copyright by do anything like this.
But, if you must, then Google is your best bet:
https://www.google.co.uk/search?q=decompile+php&aq=f&oq=decompile+php&sourceid=chrome&ie=UTF-8[
^] - but do not expect to end up with anything you can just file the serial numbers off of and sell as your own code!