3v4l.org

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<?php $arr = [ 0 => "Sat 2", 1 => "Sun 3", 2 => "Mon 4", 3 => "Tue 5", 4 => "Mon 4", ]; $arr = array_diff($arr, array_diff_assoc($arr, array_unique($arr))); print_r($arr);
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.30, 8.3.0 - 8.3.30, 8.4.1 - 8.4.21, 8.5.0 - 8.5.7
Array ( [0] => Sat 2 [1] => Sun 3 [3] => Tue 5 )

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