Assuming you installed Xampp with defaults do the following

Navigate to C:\xampp\php\ext and verify php_ldap.dll is there
Open C:\xampp\php\php.ini and find the ;extension=php_ldap.dll and remove the semi-colon so it should read extension=php_ldap.dll
Save and close the php.ini file
Stop and Restart Apache
Open http://localhost/xampp/phpinfo.php and view your php_info() to make sure the ldap section is there