使用PHP保存页面时出现禁止错误

I built a CMS that uses TinyMCE as its editor.

I have PHP protected

    protect: [
        // Protect php code
        /<\?php.*?\?>/g,
        /<\?.*?\?>/g,
    ] 

but for some reason it throws forbidden on submit (only on pages with PHP.)

I'm using an absolute URL in my form action, but it's still throwing the 403 (and gives me the relative path in the error)

I wonder if it's server side, as it works locally just fine, but not on the live server.