<?php $numbers = [4, 3, 4, 3, 1, 2, 1]; var_export( array_keys( array_intersect( array_count_values($numbers), [1] ) ) ); echo "\n---\n"; var_export( array_keys( array_filter( array_count_values($numbers), function($count) { return $count === 1; } ) ) ); echo "\n---\n"; // PHP7.4+ var_export( array_keys( array_filter( array_count_values($numbers), fn($count) => $count === 1 ) ) );
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