Back in 2009 there was a bug in the php_admin_value implementation before PHP 5.2.6. Since then, everything should run smoothly again. However, I find there's still a lot of confusion on when to use php_value, php_admin_value, php_flag and php_admin_flag. So I'll see if we can clear some of that.

He compares the three php_* flags you can use to set different kinds of values - "php_flag" for boolean operations, "php_admin_flag" (similar, but only in the vhost config) and the "php_value" that can be used for everything else, including overriding some settings in your php.ini.