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.
So in /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