Trigger function without awaiting response?

Since the program can be run from the command-line and no response is needed, using exec() does the job.

http://php.net/manual/en/function.exec.php

The first example in the comments (below) was used to run my command-line program on payment submission. The program launches and the user never awaits its completion. Shouldn’t matter if your machine is Windows or otherwise.

function execInBackground($cmd) {
    if (substr(php_uname(), 0, 7) == "Windows"){
        pclose(popen("start /B ". $cmd, "r")); 
    }
    else {
        exec($cmd . " > /dev/null &");  
    }
}

In my scenario, $cmd is ‘php54 [directory/file.php] [program parameters]’.