Basically you create a script, and you can create it as a module. Your app has to hit that module (or a cron or something if you're reading a feed), where that code gets run.
You need to call one of the ACARSData functions, to file a PIREP or update an actual flight status. To do that, you fill in a data structure and pass it to it. For example, kACARS is updating a status of a flight:
https://github.com/nshahzad/phpVMS/blob/master/core/modules/kACARS_Free/kACARS_Free.php#L179
There's that array, which get's passed to UpdateFlightData().
So perhaps it will be something like simplexml_load_data('your feed url'), and then filling out that data structure and passing it to UpdateFlighData.
FilePIREP is similar:
https://github.com/nshahzad/phpVMS/blob/master/core/modules/kACARS_Free/kACARS_Free.php#L255