Description:
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Using array_walk will crash PHP if a method of an uninstantiated object variable is used in the callback method. Other functions that take variables (at least, usort(), array_map(), array_reduce()) error out instead. There has to be at least one element in the array being walked.

Reproduce code:
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$array = array('at least one element');

array_walk($array, array($nonesuchvar,'show'));


Expected result:
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PHP 4.3.10's error message for this situation reads:

Warning:  array_walk(): Unable to call Array() - function does not exist in ...

while PHP 5.0.3 also reports a Notice-level message that $nonesuchvar is undefined.

Actual result:
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A Notice-level message that $nonesuchvar is undefined, then a Windows core dump.