Connection failed: could not find driver

Fatal error: Class 'mysqli' not found in C:Apache24htdocsconexao_db.php on line 7

<?php
    $servername = "localhost";
    $username = "root";
    $password = "minha_senha";

    try {
        $conn = new PDO("mysql:host=$servername;dbname=php_db", $username, $password);

        $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        echo "Connected successfully";
       }
    catch(PDOException $e)
    {
      echo "Connection failed: " . $e->getMessage();
    }
?>

include_path = "C:phpincludes"

extension_dir = "ext"

extension=D:/PHP/ext/php_mysql.dll
extension=D:/PHP/ext/php_mysqli.dll