Question

I want to highlight some words (ex. “true”, “false”), but only when they are between the tags <?= and ?>.

In the example below I only want to highlight the second “true” keyword:

<input checked="true"><?=($i === true ? $foo : $bar)?>

Is there any special font-locking mode for this kind of situation?

Answer 1

Font lock can call functions too in addition to matching regexps:

Find text by calling function, and highlight the matches it finds
using font-lock-keyword-face.

http://www.gnu.org/s/emacs/manual/html_node/elisp/Search_002dbased-Fontification.html

Only you need to know Elisp.