How to open a .php file

It could be one of several file types.

Cut and paste either link to a page for descriptions of each.

http://filext.com/detaillist.php?extdetail=php

http://tinyurl.com/ocj5

This particular file most likely appears to be an adobe file because somewhere along the line you selected adobe (or reader) to be the default program to open files of unknown origin.

It is possible to change the program associated with the file. Right click on the file icon. On the General tab, you'll see the program associated with that file, the program that will be launched in an attempt to open it. Click on the Change button, and try other programs. If you don't have the program to open the file, you may need to contact the sender to see if there is some additional "reader" program associated with it. or, with the link pasted above, you may be able to fond one on your own.

Try opening with notepad.

If is the source code of the php page, it will be plain text.
If not, maybe is some obscure file extension for acrobat