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<?php $episodes = [ 1 => [ ['el' => 1, 's' => 1, 'ep' => 101], ['el' => 1, 's' => 1, 'ep' => 102], ['el' => 1, 's' => 2, 'ep' => 103], ['el' => 1, 's' => 3, 'ep' => 104], ] ]; print_r(array_keys(array_column((array) $episodes[1], 's', 'ep'), 2));

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