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<?php $values = [1,2,3,1,6,1,23,2]; $result = array_keys(array_intersect(array_count_values($values), [1])); var_dump($result);
Output for 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.33, 8.2.0 - 8.2.29, 8.3.0 - 8.3.25, 8.4.1 - 8.4.12
array(3) { [0]=> int(3) [1]=> int(6) [2]=> int(23) }
preferences:
158.14 ms | 408 KiB | 5 Q