Windows IIS - PHP PDO Mysql无法找到驱动程序

Question

我有以下代码从本地PHP连接到Mysql数据库（Windows，使用IIS），

$dbhost     = "xxxx.database.windows.net";
$dbname     = "yyyy";
$dbuser     = "uuuu";
$dbpass     = "pppp";
$driver     = "sqlsrv";
$connection = new PDO("$driver:host=$dbhost;dbname=$dbname",$dbuser,$dbpass);

php信息说：

Loaded Configuration File   C:\Program Files (x86)\IIS Express\PHP\v5.6\php.ini

我在这个php.ini文件中有以下内容

[WebPIChanges]
error_log=C:\Windows\temp\PHP56ForIISExpress_errors.log
upload_tmp_dir=C:\Windows\temp
session.save_path=C:\Windows\temp
cgi.force_redirect=0
cgi.fix_pathinfo=1
fastcgi.impersonate=1
fastcgi.logging=0
max_execution_time=300
date.timezone=Europe/Minsk
extension_dir="C:\Program Files (x86)\iis express\PHP\v5.6\ext\"

[ExtensionList]
extension=php_mysql.dll
extension=php_mysqli.dll
extension=php_mbstring.dll
extension=php_gd2.dll
extension=php_gettext.dll
extension=php_curl.dll
extension=php_exif.dll
extension=php_xmlrpc.dll
extension=php_openssl.dll
extension=php_soap.dll
extension=php_pdo_mysql.dll
extension=php_pdo_sqlite.dll
extension=php_imap.dll
extension=php_tidy.dll
extension=php_sqlsrv.dll

此文件存在：C：\ Program Files（x86）\ IIS Express \ PHP \ v5.6 \ ext \ php_pdo_mysql.dll

为什么我收到“找不到驱动程序”错误？我的应用程序池可以运行32位或64位。