A freshly installed Debian etch server uses php4 as its default php engine. To changes it you have to remove the following package.
To remove them, use the following command:
# apt-get remove php4-common
# apt-get remove libapache2-mod-php4
Install then the following, php5, libapache2-mod-php5 by issuing the following command :
# apt-get install php5
# apt-get remove libapache2-mod-php5