fix php disambiguation regex (#4185)
fix php disambiguation regex (#4185)

Because of an error in the regular expression, disambiguation rule for the .php
matched any non-empty file as PHP, except for Hack files.
pchaigno/linguist,sebgod/linguist,github/linguist,github/linguist,erkyrath/linguist,christop/linguist,github/linguist,erkyrath/linguist,sebgod/linguist,erkyrath/linguist,github/linguist,erkyrath/linguist,pchaigno/linguist,pchaigno/linguist,pchaigno/linguist,christop/linguist,sebgod/linguist,sebgod/linguist