Ubuntu + Apache 2.0 + PHP 5.2.1 - Cannot Use Dynamic Extensions (e.g. Pdo.so)

I'm trying to compile PHP 5.2.1 on my Ubuntu 6.10 server. I've downloaded the source code from php.net and am running the following configure command on it:

Code:
./configure --with-apxs2=/usr/bin/apxs2 --with-mysql --with-informix=/opt/informix --enable-shared --enable-pdo=shared --without-sqlite --without-pdo-sqlite --with-pdo-mysql=shared --with-zlib --with-xml --with-pdo-informix=shared,/opt/informix
After doing this, then make, I copy over the generated libphp5.so file into my Apache extensions directory, add extension=pdo.so (and lines for the PDO-MySQL/Informix extensions) into my php.ini and restart Apache. I would expect to see PDO, PDO-MySQL and PDO-Informix listed in my phpinfo() - but they don't appear.

So, I tried recompiling PHP but without any of the PDO extensions being shared. This worked, and I could see PDO and PDO-MySQL in phpinfo(), but not PDO-Informix.

So, can anyone hazard a guess as to why none of the PDO extensions load when compiled shared? I don't get any related error messages in the Apache or System logs by the way....