When I try to install libapache2-mod-php5, I get an error:

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help resolve the situation:

The following packages have unmet dependencies:
 libapache2-mod-php5 : Depends: apache2-mpm-prefork (> 2.0.52) but it is not going to be installed or
                                apache2-mpm-itk but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I have tried apt-get clean, update and upgrade, as well as -f install, and no change.

Is there a way to fix this, or is there an easy way to manually install the php plugin?