Fix regular expression for highlighting $this, $that

Thanks phil-s.

Author: syohex

php-mode.el

@@ -1328,7 +1328,7 @@ a completion list."
      ("->\\(\\sw+\\)\\s-*(" 1 'default)
 
      ;; Highlight special variables
-     ("\\$\\(this\\|that\\)" 1 font-lock-constant-face)
+     ("\\$\\(this\\|that\\)\\_>" 1 font-lock-constant-face)
      ("\\(\\$\\|->\\)\\([a-zA-Z0-9_]+\\)" 2 font-lock-variable-name-face)
 
      ;; Highlight function/method names