How to check the installation of a PHP extension
PHP features a set of extensions which you can use on your system. To see how to enable a certain extension, check out this other tutorial, Enabling a PHP extension
From the Command Line Interface
Assuming you have access to the command line on the system where you want to check, you can simply use
php -i
This will display (among other information) information about all the installed modules. On Linux, you can use | grep to filter the output and see if it contains the one you're looking for. For isntance, if you wanted to see if "libxml" is installed, you could try:
php -i | grep libxml
On a remote server
It's a common situation that you only have access to PHP through your web server. In this case, you should read Obtaining information about your PHP Installation with phpinfo() .
Let's assume you don't want to display ALL the php information, just the loaded extensions. In this case, you could use inside the phpinfo file:
<?php phpinfo(INFO_MODULES); ?>
If you can find your module in the generated list of modules, then it's installed and available for use.