You need to make sure the number of MySQL connections (max_connections in my.cnf) is greater than the number of PHP processes ([pm.max_children][1] if you use php-fpm, or [MaxClients][2] if you're using Apache mod_php).

The default for Apache is often 300, and the MySQL default is only 100, so you can run out of connections pretty quickly if these values aren't tuned.


  [1]: http://php.net/manual/en/install.fpm.configuration.php#pm.max-chidlren
  [2]: http://httpd.apache.org/docs/2.4/mod/mpm_common.html#maxrequestworkers