Integration server OOTB (also API Gateway) can not execute/run PHP files.
You’d have to install a PHP ready webserver (e.g. Apache/lighttpd, mod_php, etc) on a server which could serve the php file, or you could run the php file from the command line (depending on the php file).
If the PHP file is in a webserver, then you can directly link it in the gateway assuming it’s reachable from the gateway end point, and/or also use the httpclient in IS to invoke it.
If it’s a command line type PHP, you could use IS to invoke the PHP file from the command line my making a shell/command invocation.