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<?php $array = [ 'a' => [ 'a' => [ 'a' => null, 'b' => null ], 'b' => [ 'a' => null, 'b' => null ] ], 'b' => [ 'a' => [ 'a' => null, 'b' => null ], 'b' => [ 'a' => null, 'b' => null ] ] ]; function getKeyPaths(array $array, string $prefix = ''): string { return implode( ',', array_reduce( array_keys($array), fn($carry, $k) => $carry + [$k => is_array($array[$k]) ? getKeyPaths($array[$k], "$prefix$k") : "$prefix$k"], [] ) ); } var_export(getKeyPaths($array));