If this is on a local server and a manual install uncommenting the line should activate it. You would need to ensure you are editing the working ini file that is being used by php. To see where the file to edit is you can make a php info page by adding this code to the top of a new or existing php page:
php:
<?php phpinfo(); ?>
In the first set of settings it should list where the active or loaded ini file is located. This is the file that you will need to update.
If this is on a live site or php was installed by an installer you will need to get support from your host or software provider to help you make this update.
