On linux I've found that I explicitly have to add the -c <path to php.ini> with the shell and
PHPIniDir="/etc/php.ini" in php5.conf or httpd.conf otherwise the ibm_db2.so is not loaded.
It doesn't help setting the --sysconfdir=/etc for configure when compiling.
% php -m | grep db2
% php -c /etc -m | grep db2
After installation of ibm_db2 on Linux, you have to add into php.ini:
Dont forget the path to ibm_db2.so. As for me it does not work without the path.