Syntax highlight

Hello! Some characters (like [, ], {, } and backslash sequences) in double quote strings are highlighted as keywords even on places where they don't have any special meaning (like [] in "foo[] $a"). I mean highlighted by `php -s` and highlight_* functions. These characters are not highlighted in constant strings (i.e. strings without variables). To behave the same in both cases and to not confuse someone by highlighting characters without special meaning, I am sending this patch. Jakub Vrana