The most common reason for the error is using an invalid PHP handler/directive in the [.htaccess] and [.user.ini] files on CloudLinux-based servers. When you access the site, it will display the following error:
Error:
"Your PHP installation appears to be missing the MySQL extension which is required by WordPress"
Or,
"PHP Warning: session_start(): open(/var/cpanel/php/sessions/ea-php56/sess_03i13, O_RDWR) failed: No such file or directory"
Note:
The exact error might be different on your website.
Solution
Fix incorrect PHP handler or session path