Go to terminal:

sudo apt-get install apache2 mysql-server php5 php5-odbc libapache2-mod-php5 php5-xsl php5-gd php-pear libapache2-mod-auth-mysql php5-mysql libmyodbc

Now hand edit (or create, if it does not exist already) /etc/odbc.ini

Here's an example odbc.ini:

[ODBC Data Sources]
odbcname     = MyODBC 3.51 Driver DSN

[odbcname]
Driver       = /usr/lib/odbc/libmyodbc.so
Description  = MyODBC 3.51 Driver DSN
SERVER       = my.database.server.com
PORT         =
USER         = USER
Password     = XXXXXX
Database     = DBNAME
OPTION       = 3
SOCKET       =

[Default]
Driver       = /usr/local/lib/libmyodbc3.so
Description  = MyODBC 3.51 Driver DSN
SERVER       = localhost
PORT         =
USER         = root
Password     =
Database     = test
OPTION       = 3
SOCKET       =

Remember to start and stop mysql and apache:

sudo /etc/init.d/mysql start
sudo /etc/init.d/apache2 start

sudo /etc/init.d/mysql stop
sudo /etc/init.d/apache2 stop
sudo /etc/init.d/mysql restart
sudo /etc/init.d/apache2 restart

There is currently an issue resulting from the fact that Apache on Mac OS X 10.5 is built as a 64 bit application. On Macs that support 64 bits (all current Mac models), Apache will run in 64 bit mode. This is incompatible with most ODBC drivers, including those currently available from Actual Technologies.
Until we can make 64 bit versions of our drivers available, we recommend a workaround to force Apache to run in 32 bit mode. Using the Terminal prompt, enter the following commands (this is for ppc machines, for intel switch ppc7400 to i386):

sudo mv /usr/sbin/httpd /usr/sbin/httpd.ub

sudo lipo -thin ppc7400 /usr/sbin/httpd.ub -output /usr/sbin/httpd.ppc7400

sudo ln -s /usr/sbin/httpd.ppc7400 /usr/sbin/httpd 

Now restart Web Sharing in the System Preferences, and continue with the instructions for "Invoking ODBC Functions from ODBC" below.
NOTE: if you ever need to restore Apache to 64 bit mode, just enter the following command:

sudo mv /usr/sbin/httpd.ub /usr/sbin/httpd