Here is how to change from apache2-mpm-prefork to apache2-mpm-worker, , when having used mpm-prefork before, and php5, on Debian Squeeze.

Step1. Stop Apache2:

# /etc/init.d/apache2 stop

Step2. Install apache2-mpm-worker:
That will remove some packages, like apache2-mpm-prefork and libapache-mod-php5, and install apache2-mpm-worker, and php5-cgi instead.

# apt-get install apache2-mpm-worker

Step3. Set Apache2 to use php5-cgi:

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