This one is pretty simple but sometimes frustrating to troubleshoot because the error below has you hunting for a file called Mail.php somewhere in your document root. Spoiler alert: it’s totally not there and never will be.
[Mon Sep 30 20:04:05.895978 2019] [:error] [pid 21698] [client 192.168.1.1:20918] PHP Warning: require_once(Mail.php): failed to open stream: No such file or directory in /var/www/website/public_html/sendmail.php on line 111, referer: https://www.website.com/ [Mon Sep 30 20:04:05.896121 2019] [:error] [pid 21698] [client 192.168.1.1:20918] PHP Fatal error: require_once(): Failed opening required 'Mail.php' (include_path='.:/php/includes:/usr/share/pear') in /var/www/website/public_html/sendmail.php on line 111, referer: https://www.website.com/
Maybe this is the first PHP mail form you’ve built on this server or you’re migrating to a new one, but in any case the solution is super easy. You need to tell pear to install Mail.
$ sudo pear install -a Mail
Your mysterious Mail.php now lives at /usr/share/pear/Mail.php.
Hat-tip to Koteshwar, the lonely 0 reputation answer on the bottom of the page. If I had 15 reps, I’d upvote your post.