If you want to fill the LiveCycle Designer form, then you need to render it with the datafile.
The datafile can be an XML file or a webservice or Database.
You cannot directly fill the PDF from a PHP script. What you can do is, have a LiveCycle Process created in Workbench and then invoke the Process from PHP application by passing the information. The Livecycle process will merge the data with the template and return the PDF to the client application.
There is an other feature in Acrobat PRO that you can import an XML file directly into a PDF but that is a manual effort. You can not link that to a PHP Script.
Thanks
Srini