Why does GNOME have to be removed for PHP 7 on Debian 8 (Jessie)?

Question

Just upgraded my VirtualBox VM's OS from Debian 7 (Wheezy) to Debian 8 (Jessie) and want to PHP from 5.5.x to 7.0.2. The simpliest solution seems to do this via the Synaptic Package Manager. So I added

deb http://packages.dotdeb.org jessie all
deb-src http://packages.dotdeb.org jessie all

to the /etc/apt/sources.list, performed apt-get update, opened the Synaptic GUI. The search by "php7" returns a list of PHP 7 packages. OK. But when I want to mark one, e.g. php7.0 for installation and see the list of the affected packages, I find a lot of packages, that would be removed, e.g. gnome!

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I've already tried it out. After this I've lost the OS GUI and even coulnd not reinstall it (and was happy to have a snapshot created before the experiments with PHP 7).

What is the problem here and how to solve/avoid it?