Error in PHP PDO Class

I'm trying to use the PDO class on one of my systems and this error is showing up, and the PDO class is already installed.

Fatal error: Class 'PDO' not found in /home/dfmedica/public_html/cursos/classes/BD.class.php on line 9

Connection class:

<?php
class BD{
    private static $conn;

    public function __construct(){}

    public static function conn(){
        if(is_null(self::$conn)){
            self::$conn = new PDO('mysql:host='.HOST.';dbname='.BD.'', ''.USER.'', ''.PASS.'');
        }

        return self::$conn;
    }
}
?>

NOTE: Bank access data is correct as I have already tested it.