Informix PDO needs the INFORMIXDIR system variable available to the script running under the webserver, in my case apache 2.4.
Using setenv did not work, so I added in the systemd startup script /lib/systemd/system/apache2.service the following:
Environment=INFORMIXDIR=/opt/Informix_Software_Bundle
in the [Service] section.
Of course, the installation directory of your informix libraries may be different than /opt/Informix_Software_Bundle....