A couple of thoughts. Let's just remove default implementation of PHPEscaper. This is not hard to define own class, in a global namespace or with 'use'.
Maybe allow to pass not limited set of arguments? E.g. like <?* $str, 'my_context', 'param', 'another_param' ?> This can be used for all context-dependent text transformations - not just for escaping, but also for translation, date and number formatting, etc. <?* $message, 'translate', $messageCategory ?> Maybe use ':' as a special sign? "<?: ?>" is more comfortable to type.
