As usual, upgrading an OS require you to reconfigure the previous installed services such as MySQL, PHP, and Apache.
There is a 64-bit version of MySQL 5.1 that you can use on Snow Leopard. Although it was designed for Leopard, you can deploy it without any hassles.
However the .pkg installer contains only 32-bit version of MySQL Preference Pane menu. Installing it will reopen the System Preferences into compatibility mode and sometimes it will bothering you.
Thanks to the people at Swoon.net that already made 64-bit MySQL prefPane. Grab it and you’re good to go.

Don’t forget to rename the php.ini.default file located in /etc/ directory to php.ini. Then, alter some lines below to be able linking PHP to the MySQL socket:
mysql.default_socket = /var/mysql/mysql.sock
mysqli.default_socket = /var/mysql/mysql.sock
Change to:
mysql.default_socket = /tmp/mysql.sock
mysqli.default_socket = /tmp/mysql.sock
If you are new to the Mac, launch Terminal.app from Applications/Utilities folder. Then type commands below to alter the file as instructed above:
cd /etc/
sudo mv php.ini.default php.ini
(you will need to enter password)
sudo nano php.ini
Scroll down using arrow key and find the mysql.default.socket line above.
To save the current changes, press control+x and type ‘y’ then enter key to proceed.
By default, Snow Leopard has built-in PHP 5.3.0. To activating it, edit the httpd.conf file located in /etc/apache2/ directory and remove the commented line below:
#LoadModule php5_module libexec/apache2/libphp5.so
Change to:
LoadModule php5_module libexec/apache2/libphp5.so
Save it.
Now start your MySQL and Web Sharing from System Preferences.