Fatal error: Uncaught Error: Undefined constant "PHPMailer\PHPMailer\FILTER_FLAG_HOST_REQUIRED" in C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php:3581 Stack trace: #0 C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php(3547): PHPMailer\PHPMailer\PHPMailer::isValidHost('www.softlib.cn') #1 C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php(2288): PHPMailer\PHPMailer\PHPMailer->serverHostname() #2 C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php(1405): PHPMailer\PHPMailer\PHPMailer->createHeader() #3 C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php(1303): PHPMailer\PHPMailer\PHPMailer->preSend() #4 C:\xampp\htdocs\v6\sem6.php(39): PHPMailer\PHPMailer\PHPMailer->send() #5 {main} thrown in C:\xampp\htdocs\v6\PHPMailer6\src\PHPMailer.php on line 3581