The other day I upgraded from mantis 2.25.7 to 2.25.8, then a couple of days later from php 7.4.33 to 8.1.26, the day after that, the system sent this email:

PHP Fatal error:  Uncaught Error: Call to undefined function PHPMailer\PHPMailer\ctype_alnum() in /usr/local/etc/apache24/mantis.example.com/vendor/phpmailer/phpmailer/src/PHPMailer.php:1798
Stack trace:
#0 /usr/local/etc/apache24/mantis.example.com/vendor/phpmailer/phpmailer/src/PHPMailer.php(1877): PHPMailer\PHPMailer\PHPMailer::isShellSafe()
#1 /usr/local/etc/apache24/mantis.example.com/vendor/phpmailer/phpmailer/src/PHPMailer.php(1640): PHPMailer\PHPMailer\PHPMailer->mailSend()
#2 /usr/local/etc/apache24/mantis.example.com/vendor/phpmailer/phpmailer/src/PHPMailer.php(1471): PHPMailer\PHPMailer\PHPMailer->postSend()
#3 /usr/local/etc/apache24/mantis.example.com/core/email_api.php(1416): PHPMailer\PHPMailer\PHPMailer->send()
#4 /usr/local/etc/apache24/mantis.example.com/core/email_api.php(1233): email_send()
#5 /usr/local/etc/apache24/mantis.example.com/core/email_api.php(2123): email_send_all()
#6 [internal function]: email_shutdown_function()
#7 {main}
  thrown in /usr/local/etc/apache24/mantis.example.com/vendor/phpmailer/phpmailer/src/PHPMailer.php on line 1798

I'm guessing it's due to the PHP update.

Look like mantis 2.25.8 has PHPMailer 6.5.0. I see manits 2.26 updated to PHPMailer 6.8.0 in 0029025. I suppose I'll try updating mantis to 2.26.

But this appears to mean that 2.25.8 is not fully PHP 8 compatible...