Online PHP editor | refs for cAPBT

<?php $firstArr = [ ['id' => 'old_id_1', 'color' => 'red'], ['id' => 'old_id_2', 'color' => 'blue'], ['id' => 'old_id_3', 'color' => 'green'], ['id' => 'old_id_4', 'color' => 'purple'], ['id' => 'old_id_5', 'color' => 'yellow'], ]; $secondArr = [ ['id' => 'new_id_1', 'color' => 'blue'], ['id' => 'new_id_2', 'color' => 'pink'], ['id' => 'new_id_3', 'color' => 'purple'], ]; $firstArrAssoc = array_combine(array_column($firstArr, 'color'), array_column($firstArr, 'id')); $duplicatesArr = []; foreach ($secondArr as $row) { if (array_key_exists($row['color'], $firstArrAssoc)) { $duplicatesArr[] = "- $row[color] ($row[id]) in {$firstArrAssoc[$row['color']]}"; } } echo "We find " . count($duplicatesArr) . " duplicates colors :\n"; echo implode("\n", $duplicatesArr);