Since to the patch of php bug #43227 There is no more effect on .htaccess ‘php_value mbstring.func_overload’ setting. The work-around is to set func_overload into Apache virtualhost config file. php_admin_value mbstring.func_overload X X is the value that you want to set.
You may use .htaccess to enable php feature by php_value output_buffering On However, If the php is not running as Apache modules, 500 errors probably come up. In this case you should use this instend. php_flag output_buffering On