One of the challenges of this [three tier server setup] is where and how to store the connection information for all these services. We have done several things in the past. The most common thing is to store this information in a PHP file. [...] We have taken [it] one step further using some PHP ini trickeration. We use ini files that are loaded at PHP's startup and therefore the information is kept in PHP's memory at all times.

They use the get_cfg_var function and the "--with-config-file-scan-dir" option to tell PHP to automatically load in the ini files it finds in the named directory. He gives an example of both a simple configuration and a more complex situation where a MySQL instance can read from the ini file containing the username/password/host information.