solved qlo kasusnya :
Array
(
    [5] => Array
        (
            [0] => RT XI
        )

    [4] => Array
        (
            [0] => RT IX
            [1] => RT X
        )

    [3] => Array
        (
            [0] => RT III
        )

    [2] => Array
        (
            [0] => RT II
            [1] => RT I
        )

    [1] => Array
        (
            [0] => RT V
            [1] => RT IV
        )

)

tapi jika :
Array
(
    [7] => Array
        (
            [0] => RT X
        )

    [5] => Array
        (
            [0] => RT XI
        )

    [4] => Array
        (
            [0] => RT IX
        )

    [3] => Array
        (
            [0] => RT III
        )

    [2] => Array
        (
            [0] => RT II
            [1] => RT I
        )

    [1] => Array
        (
            [0] => RT V
            [1] => RT IV
        )

)
outputnya :
RT X
RT XI
RT IX
RT III
RT II, RT I2
harusnya
RT X
RT XI