Original array:
Array
(
    [0] => Array
        (
            [foo] => 23
            [bar] => a
        )

    [1] => Array
        (
            [foo] => 666
            [bar] => b
        )

    [2] => Array
        (
            [foo] => 23
            [bar] => c
        )

    [3] => Array
        (
            [foo] => 23
            [bar] => d
        )

    [4] => Array
        (
            [foo] => 666
            [bar] => e
        )

    [5] => Array
        (
            [foo] => 55
            [bar] => f
        )

    [6] => Array
        (
            [foo] => 23
            [bar] => g
        )

    [7] => Array
        (
            [foo] => 55
            [bar] => h
        )

)
New array:
Array
(
    [23] => Array
        (
            [0] => a
            [1] => c
            [2] => d
            [3] => g
        )

    [666] => Array
        (
            [0] => b
            [1] => e
        )

    [55] => Array
        (
            [0] => f
            [1] => h
        )

)