PHPからMySQL 8.0へPDOで接続時に［could not find driver］になる場合の対処法

<?php
try
{
  $pdo = new PDO('mysql:host=localhost;dbname=db', 'administrator', 'password', array(PDO::ATTR_EMULATE_PREPARES =&amp;gt; false));
}
catch (PDOException $e)
{
  exit('データベース接続失敗。'.$e->getMessage());
}
?>

; Directory in which the loadable extensions (modules) reside.
; http://php.net/extension-dir
; extension_dir = "./"
; On windows:
; extension_dir = "ext"

; Directory in which the loadable extensions (modules) reside.
; http://php.net/extension-dir
; extension_dir = "./"
; On windows:
extension_dir = "ext"

; Notes for Windows environments :
;
; - Many DLL files are located in the extensions/ (PHP 4) or ext/ (PHP 5+)
;   extension folders as well as the separate PECL DLL download (PHP 5+).
;   Be sure to appropriately set the extension_dir directive.
;
; extension=pdo_mysql

; Notes for Windows environments :
;
; - Many DLL files are located in the extensions/ (PHP 4) or ext/ (PHP 5+)
;   extension folders as well as the separate PECL DLL download (PHP 5+).
;   Be sure to appropriately set the extension_dir directive.
;
extension=pdo_mysql