for c-symbol-key.
(php-mode): Add underscore (_) to syntax table for
font-lock-defaults.
(php-font-lock-keywords-1, php-font-lock-keywords-2)
(php-font-lock-keywords-3): Change references to \\s_ in regular
expressions to just using \\sw. Remove usage of "[^_$]?" and
"[^_]?" regular expression.
