Why did my PHP5.5 upgrade fail?

Why did my PHP5.5 upgrade fail?

I have a relatively new install of Ubuntu 12.04. The php version is 5.3,
so I just followed this question to upgrade to 5.5: How do I install
latest PHP in supported Ubuntu versions (like 5.4.x in Ubuntu 12.04)?
When I ran the final command though (sudo apt-get upgrade), it gave me the
following output:
Reading package lists... Done Building dependency tree Reading state
information... Done The following packages have been kept back: apache2
apache2-mpm-prefork apache2.2-bin libapache2-mod-php5 libjson0 php5-cli
php5-common php5-mysql 0 upgraded, 0 newly installed, 0 to remove and 8
not upgraded.
Why does it 'keep packages back'?