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<?php $array = [ ["id" => 21153, "genre" => ["History", "Drama", "Thriller"]], ["id" => 21152, "genre" => ["ACTION"]], ["id" => 21151, "genre" => ["ROMANTIC"]], ["id" => 21150, "genre" => ["Drama"]] ]; $flat = array_merge(...array_column($array, 'genre')); var_export( array_combine($flat, $flat) );
Output for 7.4.0 - 7.4.33, 8.0.1 - 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 ( 'History' => 'History', 'Drama' => 'Drama', 'Thriller' => 'Thriller', 'ACTION' => 'ACTION', 'ROMANTIC' => 'ROMANTIC', )

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109.12 ms | 407 KiB | 5 Q