Evidence from another machine I administer (same version of OS and unattended-upgrades and that uses a similar PHP configuration) suggests that the bug may only manifest if the previous state was 'un' rather than 'rc':
$ dpkg -l libapache2-mod-php5|grep ^[a-z] rc libapache2-mod-php5 5.3.10-1ubuntu3.6 i386
on this machine, unattended upgrades did not attempt to install libapache2-mod-php5.
Evidence from another machine I administer (same version of OS and unattended-upgrades and that uses a similar PHP configuration) suggests that the bug may only manifest if the previous state was 'un' rather than 'rc':
$ dpkg -l libapache2- mod-php5| grep ^[a-z]
rc libapache2-mod-php5 5.3.10-1ubuntu3.6 i386
on this machine, unattended upgrades did not attempt to install libapache2- mod-php5.