Informix

Informix is a family of relational database management system (RDBMS) products by IBM.

unixODBC
In order to connect to the Informix database, the unixODBC package must be installed. This provides a standardized API for using dabase management systems.

IBM Informix CSDK
Download the SDK from IBM's website IBM Informix Downloads. While it is downloading. Start setting up the required user and group accounts.

Untar the file (I downloaded the x86_64 version of the CSDK, file names may differ)

This produces four files, csdk.ini, csdk.jar, doc (dir), and installclientsdk. Run the installer.

During the set up, change any options necessary, accepting all of the default options does not appear to hinder any usability.

Configuration
There are five files that need to be modified. Please note that a lot of the settings in sqlhosts and /etc/services will probably satisfy your particular requirements. Please consult the installation guide of your informix server in order to determine the proper settings.

Setting up environment variables
Informix requires certain variables to be set, do so in the following file.

Then run the following:

sqlhosts
Edit the sqlhosts file to contain your server's name, nettype, hostname/IP and the service name.

Services
Append your services file to contain the services_name listed above followed by the port and protocol type.

Testing
Connect to your database using the following command

Database not found
Depending your particular installation of Informix; you may to specify the absolute path the database, instead of just the name of the database. You will get an error similar to this if the database is not properly specified. The error will look similar to the following: [Informix][Informix ODBC Driver][Informix]Database not found or no system permission. If that is the case then change your Database line in odbc.ini to reflect the exact location.

Missing Libraries
Chances are, your LDPATH isn't completely set up. Use ldd to determine which libraries aren't accessible.

Resources

 * Gentoo Forum post: HOWTO Informix + PHP
 * Blog Post regarding "database not found"