This extension has been moved to the » PECL repository and is no longer bundled with PHP as of PHP 5.2.1
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » https://pecl.php.net/package/informix.
To be able to use the functions defined in this module you must compile your PHP interpreter using the configure line --with-informix[=DIR], where DIR is the Informix base install directory, defaults to nothing.