You can use Mendix deeplinks to set up unique links into your mendix system that can pass data as a GET parameter to your microflows.
If you want to use a POST method call into the mendix system you can add your own custom request handler that could accept post requests with the data you want.
As Sebastiaan mentioned there are a number of options. Let us know what sort of thing you are looking for and give you some better suggestions.
Why not build the basic forms in Mendix?
You could use web services to get data from one data source to another:
You could also export the data from the PHP site to Excel and then import this into the mendix application.
There are many other possibilities, but in the first place, the main purpose for using Mendix is that you don't have to build websites in PHP/HTML anymore :)