I am starting a fm script via php.

Php is now waiting until the fm script has been completed but I do not need that - is there any way to tell php that it should continue with its own script?

Not directly no...

You might be able to simulate your desired behaviour using either the exec call and executing your file using the php.exe and it's command line support or you can open a curl based request to the page with options set to drop the connection once it's established...

Either way, it's a bit of a hack and probably not a good idea to abandon the script after you've started it.