Try:
find . -name '*.php' | xargs wc -l
or (when file names include special characters such as spaces)
find . -name '*.php' | sed 's/.*/"&"/' | xargs wc -l
The SLOCCount tool may help as well.
It will give an accurate source lines of code count for whatever
hierarchy you point it at, as well as some additional stats.
Sorted output:
find . -name '*.php' | xargs wc -l | sort -nr