I was bitten by this lately, so I post it here as a hint (it may bite you too):
You use apt on your Debian/Sarge to install apache2, mysql, php4 and of course you also install php4-mysql. One should expect that after:
apt-get install php4-mysql
you can connect immediately from your php script to a MySQL database. Calling simply phpinfo() will prove you wrong because of a minor detail: You must add the following line
extension=mysql.so
at /etc/php4/apache2/php.ini and then issue an apache2ctl graceful .
Now you are OK.
Managing organized complexity 
Thanks for the tip, sometimes the easiest turn into a problem, I hoped apache would re-read php.ini with apache2ctl -k restart -t, but it didn’t.