Custom httpd error page

Having long nights finding solutions to things that worked in CentOS7 but not in Rocky.

In CentOS7 in httpd.conf I set ErrorDocument 404 /error.php to display a custom error page, which worked for all non-existing files.

In Rocky I set things up the same and the custom error page displayed for all missing files, except for missing php files. In place of the custom page was “file not found.”

I found a post that said to add “ProxyErrorOverride On” in the conf.d/php.conf file, which worked. There was no explanation as to why to do this.

Can someone explain why? Thanks