The Zend Developer Zone has a new article that will interest developers still looking to make the jump from PHP4 to PHP5 (you know who you are) with Informix support. Zend Core can help.
There are many business web applications written in PHP4 that use the Informix module for data access. PHP4 is no longer supported and as a result will not be issued any more fixes or releases. Since version 5.2, the Informix module is no longer available for the PDO module. In this article I'll to describe how to compile and install the Informix module on the Zend Core 2.5 (PHP 5.2.6)
The tutorial helps you set up your environment and build/install the Informix module to be plugged into the Zend Core stack. Activation is as simple as going into the Zend Core admin tool and enabling the extension. A sample script is included to test the results.
Its also noted in the comments that there's a PDO driver for Informix that can be pulled from the PECL extension repository.