Plesk site on CloudLinux fails to load MySQL PHP extension: Your PHP installation appears to be missing the MySQL extension

Symptoms

• A website hosted on Plesk on CloudLinux shows the following error

  Your PHP installation appears to be missing the MySQL extension which is required by WordPress

• The mysql extension is enabled in PHP Selector, both over SSH and in Domains > example.com > PHP Selector

  # selectorctl --list-extensions --version=5.6 | grep mysql
  + mysql

  However, it is not enabled when checking CloudLinux PHP directly

  # /opt/alt/php56/usr/bin/php-cgi -m | grep mysql
  (Empty Output)

• The mysql library exists on the server, and its extension is enabled

  # ls -la /opt/alt/php56/usr/lib64/php/modules | grep mysql
  lrwxrwxrwx 1 root root 36 Jul 5 13:55 mysql.so -> /opt/alt/php56/etc/mysql5.5/mysql.so

  # egrep -Ri mysql /opt/alt/php56/etc/php* | egrep ^extension
  /opt/alt/php56/etc/php.d.all/mysql.iniextension=mysql.so

• The solution from this article doesn't work: Unable to install WordPress using WP Toolkit: Call to undefined function json_encode()

Cause

A misconfiguration in CloudLinux PHP. Not all the packages required by Alt-PHP are installed.

Resolution

Contact CloudLinux Support to get assistance with their PHP versions.