Add the source lists:
deb http://snapshot.debian.org/archive/debian/20120221T041601Z/ unstable main
deb-src http://snapshot.debian.org/archive/debian/20120221T041601Z/ unstable main
Now update apt but ignore expiry dates:
apt-get -o Acquire::Check-Valid-Until=false update
Now you can pass in the version number you need 5.3.10-2 in this case
apt-get install php5=5.3.10-2
http://superuser.com/questions/913792/php-5-3-10-on-debian-jessie