&reftitle.install; To build the PDO_INFORMIX extension, the Informix Client SDK 2.81 UC1 or higher must be installed on the same system as PHP. The Informix Client SDK is available from the IBM Informix Support Site. PDO_INFORMIX is a PECL extension, so follow the instructions in to install the PDO_INFORMIX extension. Issue the configure command to point to the location of your Informix Client SDK header files and libraries as follows: The configure command defaults to the value of the INFORMIXDIR environment variable.