Typically you would
Type default programs into the search box and hit enter
Select "Associate a file type or protocol with a program" from the resultant page
Then scroll to .php select it and click the Change Program button at the top right use the browse button to find the program you want to associate with that extension.
Failing this perhaps refer to this article for using the ASSOC command from an elevate command prompt it may help How to Unassociate File Types and Remove File Extension Associations in Windows Vista