Description:
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Same problem as with bug #45555 - just for a different extension: The interbase extension accepts arbitrary zvals as callbacks (which is fine) but always uses Z_STRVAL(zval) to display an error message if the callback is invalid. This will cause a segfault if e.g. int(1) is passed as a callback.

The following patch against PHP_5_3 (it also applies against PHP 5.2) solves the problem (there are two places where error messages are generated):

http://www.christian-seiler.de/temp/ibase-5.3.patch

And the same patch ported to HEAD:

http://www.christian-seiler.de/temp/ibase-6.patch

Please note that I only found this bug because I did a source review of all core extensions that call callbacks in order to ensure they are compatible with closures. I have no knowledge or experience whatsoever with Interbase or Firebird so I can't actually test this. But by reading the source it is clear that this will crash PHP (see bug #45555 where the same problem occurred but where I could test the issue).

Reproduce code:
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Probably something like:
ibase_set_event_handler($connection, 1, "whatever");

Expected result:
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Callback argument 1 is not a callable function

Actual result:
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Segmentation fault.