When using PDO_INFORMIX php driver with an Apache Web Server, the httpd process receives a -25546 error on a connection attempt. Subsequent connection attempts will continue to receive the error until Apache is restarted.
-25546 Invalid network association control
The cause of the problem is using the event model with Apache. The event Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing work to supporting threads, freeing up the main threads to work on new requests.
Resolving the problem
To resolve the problem, one must use the Apache prefork model. This is a non-threaded implementation much like Apache 1.3 in that it avoids threading for compatibility with non-thread-safe libraries.