Error in PHP PDO Class

Question

Guys I’m trying to use the PDO class in one of my systems and this error is appearing, 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;
    }
}
?>

OBS: The data of access to the bank are correct, because I have already tested.