Fix highlighting of "as" keyword and what follows

Add test case for issue #178.

The fix for "as" involves changing the order of patterns within
php-font-lock-keywords-3. Inline comments should make clear why it's
done this way and where to add new patterns. To summarize,
php-font-lock-keywords-3 is constructed out of three joined lists:

 1. php-mode patterns that apply faces before cc-mode patterns have to
    chance to apply a different face

 2. the actual detailed cc-mode patterns

 3. all other php-mode patterns, they only take effect if the matches are
    not already fontified