Any server side language has a parsing engine. To view a PHP page in AIR would require adobe to include the PHP engine inside air. Then they'd have to include coldfusion and ASP and ruby and perl etc.
understandable. but then they could just include it as a per project bases plugin. they dont have to package it up within the air download. The person who creates a app using php would include it in there app. or it could possibly see if there was an engine app already installed in the air directory and just use that with its own db file and if there is not one it would install it from the app that they want to use.
As answered: there is no pluggable add-ons - including PHP module/parser/server.
There is even place @ Adobe for discuss/submit such requests:
For now you could be forced to use 3rd party solution (open sourced):