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<?php $arr = [ 0 => "Sat 2", 1 => "Sun 3", 2 => "Mon 4", 3 => "Tue 5", 4 => "Mon 4", 5 => "Mon 4", 6 => "Sun 3", ]; $condition = function($count){ return $count == 1; }; $result = array_keys(array_filter(array_count_values($arr),$condition)); var_dump($result);

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