How to determine if PDO is enabled in PHP?

Check if the class exists:

if (class_exists('PDO'))

I appreciate the support and all the upvotes I still get, but please check Salman Abbas's answer for the proper way to do this.

Just run the command as php -m from the command prompt which will display list of modules installed for PHP

The proper way of determining that will be using the extension_loaded function:-

if ( extension_loaded('pdo') ) {
    .......
}

And you might also want to check for the database-specific PDO driver using:-

if ( extension_loaded('pdo_<database type here>') ) { // e.g., pdo_mysql
    .......
}