Thanks for the pointers here @Rinus-van-Weert! I've managed to get PHP up and running without any fuss at all. Only last thing I'd add is whilst you're in modifying uhttpd you might want set up a secondary site.
/etc/config/uhttpd I've added the following below the main config definition
... config uhttpd 'myapp' list listen_http '0.0.0.0:4000' list listen_http '[::]:4000' list interpreter '.php=/usr/bin/php-cgi' option home '/www/myapp' option cgi_prefix '/cgi-bin' option index_page 'index.php' option error_page '/oopsie.html' ...
The benifit (for me) being that, if you wanted to, you can access this site from the wider world without exposing the console.
Happy Hacking all