Where is php located in linux?

I have installed Apache and PHP. I know PHP works as I have tested a simple PHP file on an Apache server.

I'm writing a simple webserver which should be able to process PHP files. So once I get a request for a PHP file, I want to do something like 'exec php test.php' and get the output and pass it to the client.

As I'm not much into Ubuntu, I don't know where the PHP executable is [should be in \bin right?] to do it. But there is no PHP file inside \bin or \usr\bin.

When I run 'which php' it shows nothing. How do I do this?