How to enable event MPM Apache 2.4 on Ubuntu 14.04 with thread safe PHP?

To disable event MPM, you can do sudo a2dismod mpm_event and after that to enable prefork, you should do sudo a2enmod mpm_prefork.

Finally restart apache:

sudo service apache2 restart

This worked for me.

First check if you have this package installed:

dpkg -l | grep libapache2-mod-php5

If you have it just remove it:

apt-get remove libapache2-mod-php5

You can install multiple ways for Apache to communicate with PHP in parallel. Maybe you have (still) the old mod_php installed, additionally to fastcgi/php-fpm.

You can see which modules your Apache has loaded at runtime with

apache2ctl -M

If needed, use sudo to get root privileges. If you see php5_module (shared) among the modules your Apache is currently using, the old mod_php is still installed and blocks you from using mpm_worker or mpm_event.