If a php program is more than 100 lines long or so vim does a very poor
job with syntax highlighting. vim will "lose it's place" and stop
highlighting correctly. Changing the /usr/share/vim/vim57/syntax/php3.vim
file like this helps greatly:

@@ -208,7 +208,7 @@
 if exists("php3_minlines")
   exec "syn sync minlines=" . php3_minlines
 else
-  syn sync minlines=100
+  syn sync minlines=500
 endif
 
 if !exists("did_php3_syntax_inits")