When working with Apache 2.4 on Ubuntu 14.04, you might encounter issues when trying to switch between Multi-Processing Modules (MPM). The error occurs because Apache only allows one MPM module to be active at a time.

First, verify which MPM module is currently active:

apache2ctl -M | grep mpm

To switch from mpm_event to mpm_prefork:

sudo a2dismod mpm_event
sudo a2enmod mpm_prefork
sudo service apache2 restart

Ensure you have the necessary packages installed:

sudo apt-get install apache2-prefork-dev apache2-mpm-prefork

When compiling PHP from source, use the correct apxs path:

./configure --with-apxs2=/usr/bin/apxs2 --with-mysql --with-curl
make
sudo make install

If you see "Module mpm_prefork_module does not exist", it means:

1. The package isn't properly installed
2. The module file is missing
3. There's a path configuration issue

Verify the module exists:

ls /usr/lib/apache2/modules/mod_mpm_prefork.so

After making changes, always verify your configuration:

sudo apache2ctl configtest
sudo service apache2 restart

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When setting up PHP 5.4 with Apache 2.4 on Ubuntu 14.04, many developers encounter MPM module conflicts that prevent proper prefork configuration. The key error messages typically include:

module mpm_event_module is already loaded, skipping
ERROR: Module mpm_prefork_module does not exist!

Apache 2.4 on Ubuntu defaults to mpm_event_module, which conflicts with PHP's requirement for prefork. You'll need to:

1. Disable event module
2. Enable prefork module
3. Properly configure PHP

First, check currently enabled modules:

apache2ctl -M | grep mpm

Then disable the conflicting module:

sudo a2dismod mpm_event

Now enable prefork (note the correct module name):

sudo a2enmod mpm_prefork

The proper way to install PHP 5.4 from source:

./configure --with-apxs2=/usr/bin/apxs2 \
--with-mysql \
--with-curl \
--enable-mbstring \
--with-zlib

make
sudo make install

Instead of editing apache2.conf directly, create a proper PHP config file:

sudo nano /etc/apache2/mods-available/php5.conf

Add these contents:

<FilesMatch "\.ph(p[345]?|t|tml)$">
    SetHandler application/x-httpd-php
</FilesMatch>
<FilesMatch "\.phps$">
    SetHandler application/x-httpd-php-source
</FilesMatch>

After making all changes, verify your setup:

sudo apache2ctl configtest
sudo systemctl restart apache2

Create a test PHP file:

sudo nano /var/www/html/info.php

With content:

<?php phpinfo(); ?>

If you still encounter issues:

• Check Apache error logs: tail -f /var/log/apache2/error.log
• Verify module paths: ls /usr/lib/apache2/modules/
• Ensure proper permissions on PHP files