Comment 9 for bug 1241498

Judging from the output from libapache2-mod-php5filter...

----
  php5_invoke: Enable module mysqli for apache2filter SAPI
  php5_invoke: Enable module pdo_mysql for apache2filter SAPI
  apache2_invoke: Enable module php5filter
   * Restarting web server apache2
----

In /var/lib/dpkg/info/libapache2-mod-php5.postinst...

----
# Automatically added by dh_apache2
if php5_enable; then
        if [ -e /usr/share/apache2/apache2-maintscript-helper ] ; then
                . /usr/share/apache2/apache2-maintscript-helper
                for conf in php5 ; do
                        apache2_invoke enmod $conf || exit $?
                done
        fi
fi
# End automatically added section
----

Since php5_enable() always produce an error msg before it exit with status 1, and apache2_invoke was never called, it looks like the only culprit would be "for conf in php5" returned nothing.

It looks like I previously have apache2 and mod-php5 installed and I moved the entire /etc/apache2 elsewhere to solve a previous apache2 upgrade problem, so that might have contributed to it. Looks to me /etc/apache2/mods-available/php5.{conf,load} weren't there and normal remove/reinstall doesn't check for it to put them back. purge/reinstall fixed it.