That originates from php.ini max allowed memory size. But the tried to allocate thingy looks like a memory overflow.
Before digging into deep here, create a small standalone PHP script which incorporates the PDO calls to DB2 and analyse how the general flow works. I could imagine that due to some version or binary mismatch, you run into memory leaks. I’ve seen such in the past with other PDO extensions.
Cheers,
Michael