Description:
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When apcu_fetch is called with an array containing a reference to a string it fails with the following message:

Warning: apcu_fetch(): apc_fetch() expects a string or array of strings. in php shell code on line 1

Test script:
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$keys = [ 'a', 'b' ];
array_walk($keys, function(&$item) {});
apcu_fetch($keys);

Expected result:
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I expect to get an array with found keys and values.