Ticket #3920: php.syntax - highlight keyword 'null'

Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>

misc/syntax/php.syntax

@@ -35,6 +35,7 @@ context default
     keyword whole include brightmagenta
     keyword whole include_once brightmagenta
     keyword whole new brightmagenta
+    keyword whole null brightmagenta
     keyword whole private brightmagenta
     keyword whole protected brightmagenta
     keyword whole public brightmagenta