This launches the command $cmd, redirects the command output to $outputfile, and writes the process id to $pidfile.

exec(sprintf("%s > %s 2>&1 & echo $! >> %s", $cmd, $outputfile, $pidfile));

This lets you easily monitor what the process is doing and if it’s still running.

function isRunning($pid){
    try{
        $result = shell_exec(sprintf("ps %d", $pid));
        if( count(preg_split("/\n/", $result)) > 2){
            return true;
        }
    }catch(Exception $e){}

    return false;
}

 Source

Manual:

exec

proc_open