As soon as PHP 5.4.0 came out, I noticed that the native DB2 driver does not compile anymore.
The reason are changes in the Zend API.
Until a new package is released, you can use the following patch to compile the driver: ibm_db2.c.patch
Here is the entire file, just in case: ibm_db2.c
Update: A new package has been released on PECL, which includes the changes to compile the driver with PHP 5.4.