I'm not sure about the n900 but for the above error what I would do is:

- Delete php5-common_5.2.6-6maemo5_armel.deb from /var/cache/apt/archives/

- Do apt-get install -f after the above

If it still complains about the Post-removal script, do:

dpkg --remove --force-remove-reinstreq php5-common


If still the post-removal script error comes then delete the post-removal script from:

/va/lib/dpkg/info/


The above is what I think should fix it up...