answer me fez ir ler a manpage e você pode especificar a versão ao baixar a fonte com o apt-get.
Na página de manual do apt-get .
source
...
A specific source version can be retrieved by postfixing the source name with an equals and then the version to fetch, similar to the mechanism used for the package files. This enables exact matching of the source package name and version, implicitly enabling the APT::Get::Only-Source option.
Note that source packages are not tracked like binary packages, they exist only in the current directory and are similar to downloading source tar balls.
então rodando
apt-get source php5=5.3.2
deve fazer o truque