setting php.ini's variables on .htaccess

Hi, reading a previuos message, I've seen that could be possible to modify any variables in the php configuration file using .htaccess (Apache) ... but when I try to modify a simple setting variable, the server response me with an Internal Error Server ..... I mean, try to using this: php_value auto_append_file /path/to/file the response is Error etc. etc. Could anyone help me please? Many thanks in advance max