If you have root access to the server you can modify the php initialization file. If you don't have root access you can create a php initialization file and upload it to the document root of the site/hosting account you want to configure. The name of the file depends on your server. You can find more info on php's website. php.net
If you're on standard linux hosting, the name of the file is php.ini.
If your needs are specific to a single file, there is a PHP directive that changes settings for a single script. You can use ini_set to set specific configuration settings like
This is often preferable to making sitewide changes.
If you have a dedicated server, there is another (somewhat complex) way to go about it through the WHM.
I just had to do this last night. I had a piece of software that relied on the mb_internal_encoding function which isn't installed by default on the dedicated servers.
Here's what I did...
You'll need to restart Apache after this so you'll have a few seconds of downtime but this added the missing module and now the software is happy.
Another option is the PHP Configuration Editor (in the find box, type PHP and choose if from the filtered list). The basic mode gives access to things like max execution time and memory limit. The advanced mode accesses quite a few options, many of which you don't want to mess with unless you know what you're doing.