Description:
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Under some circumstances is_callable() throws a fatal error when testing if the PHP4-style constructor of the class (e.g. is_callable(array('test1', 'test1')).

Unfortunately I wasn't able to create a self-contained example, but the issue seems to happen often when using Simpletest Mocks in the OpenX test suite.

It's ok if is_callable returns false if the method cannot be called statically, but definitely a fatal error isn't expected.

Reproduce code:
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echo "Test";
is_callable(array('OX_Dal_Maintenance_Statistics', 'OX_Dal_Maintenance_Statistics'));

Expected result:
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Test

Actual result:
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Test
Fatal error: Non-static method OX_Dal_Maintenance_Statistics::OX_Dal_Maintenance_Statistics() cannot be called statically in ...