There is PHP parser written in PHP [0], a PHP VM implementation (running PHP code with PHP) [1] and a AOT compiler for PHP which is written in PHP, for a subset of the PHP language [2]. So yeah, it is already happening.

[0] https://github.com/nikic/PHP-Parser

[1] https://github.com/ircmaxell/PHPPHP

[2] https://github.com/google/recki-ct

Also https://github.com/google/recki-ct

Or spent time trying to improve PHP.