find . -name "*.php" -exec grep -H "some_string" {} \;

find in this folder (.) 
for files matching the filter ( -name ) *.php 
[ recursiveness should be default ]; 
when each file is found, execute grep, 
look for 'some_string', 
and pass in the found filename ( where the {} placeholder is ). 
Because that effectively executes grep once per file ( rather than once for many files ), you need the -H option to force grep to turn on the output of the filename; it wouldn't bother otherwise for a single file grep.