When enabling HTTP Authentication on Zabbix Frontend, I noticed that the index.php will look for the $_SERVER['PHP_AUTH_USER'] in order to know if the user is logged in and which authorizations it should have.
However, the PHP_AUTH_USER variable will not be availabled if:

• PHP SafeMode is on
• HTTP Digest Authentication

I'm not a web developer and I don't know what are the implications, but using the $_SERVER['REMOTE_USER'] instead of $_SERVER['PHP_AUTH_USER'] will solve both the issues and probably some other.
There are similar bug requests like: ZBX-3779, ZBX-6260, ZBX-5513

Attached a simple tested patch