Custom commands

[Richard Verbrugge]

I am trying to work with presets to accomplish a simple task.
Here is the precondition:
PPPoE username = provision

I'd like to update the device configuration with the new username and
password that match the serial number for this device from a database.

For configuration I want it to query a php file (which connect to an
external db): http://location/query.php?serial=SN_OF_DEVICE.

Something like this:
credentials = {result from http://location/query.php?serial=SN_OF_DEVICE'}
(= summary.pppoeusername=user324&summary.pppoepassword=234sfw)
Next it should set the 
Set summary.pppoeusername to credentials["summary.pppoeusername"]
Set summary.pppoepassword to  credentials["summary.pppoepassword"]

In a step by step guide (details are most welcome for learning!) how can
this be done?

Thank you!