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.