<?php $A = [3,45,67,8]; $B = [ 1 => [1,6,8], 2 => [5,67,3,4,5,66,6,45], 3 => [55,56,57,58], 4 => [45,80,81,82] ]; foreach ($B as $key => $haystack) { foreach ($haystack as $hay) { $C[$hay][] = $key; } } var_export(array_keys(array_flip((array_merge(...array_intersect_key($C, array_flip($A)))))));
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