If you are using suphp, try
1) adding this to your .htaccess
Code:
<Files ".ht*">
deny from all
</Files>
suPHP_ConfigPath /home/username
2 then create a php.ini file at /home/username/php.ini with this
Note: This may be disabled by your host. If it does work, you may have to add other things to your php.ini file as it will completely over-ride the server-wide one, not just add to it.