Hi,
originally we were also planning to have PHP snippets. There is still some commented code in the plugin with an unfinished implementation for such a feature. We stopped because of security concerns.
If somebody manages to write CSS or JS on your website without your permission, that is bad enough and it would hurt the website’s visitors, but it’s easy to mitigate: only delete the CSS and JS codes. On the other hand, if somebody manages to write PHP codes without your permissions, then the website is entirely compromised, as well as all the other websites on your server.
Of course we are careful about possible vulnerabilities and implemented all the WordPress recommendations and best practices in order to keep the websites free from being hacked. In case there is a vulnerability found in the future, then allowing PHP codes to be executed creates a risk too high to justify such a feature.
I know there are other plugins that allow PHP snippets on the website. It is their choice to handle such risks. We chose to limit the plugin to CSS, JS and HTML codes.
thanks for sharing your approach.