You will generally not be able to enable/disable things in the PHP configuration on a shared host. There are a number of things that can often be tweaked with settings in .htaccess files depending on your host's setup, but enabling modules/libraries is not one of them.
Most hosts do not run entirely with the default PHP options for one reason or another, so even though it is included by default in the standard distribution you can not rely on finding in included on any given host.
You need to contact your host. They may be willing to enable the feature for you if it is something that might be useful to their customers more generally, or it may be something they already have enabled on some hosts (but have more rolled out to all yet) in which case they could move your account to a server that has it already.