split array into groups

I have an array with the following:

Array
(
    [0] => Array
        (
            [PropertyID] => 248
            [UserID] => 28
        )
    [1] => Array
        (
            [PropertyID] => 254
            [UserID] => 28
        )
    [2] => Array
        (
            [PropertyID] => 247
            [UserID] => 28
        )
    [3] => Array
        (
            [PropertyID] => 257
            [UserID] => 30
        )
    [4] => Array
        (
            [PropertyID] => 250
            [UserID] => 4
        )
    [5] => Array
        (
            [PropertyID] => 252
            [UserID] => 3
        )
    [6] => Array
        (
            [PropertyID] => 255
            [UserID] => 4
        )
    [7] => Array
        (
            [PropertyID] => 246
            [UserID] => 31
        )
)

I want to split the array into groups based on the UserID values so my array would output as:

Array
(
    [0] => Array
          (
              [28] => 248
            [28] => 254
            [28] => 247
        )
    [1] => Array
        (
            [30] => 257
        )
    [2] => Array
          (
                [4] => 250
                [4] => 255
            )
      [3] => Array
            (
                  [3] => 252
            )
      [4] => Array
            (
                  [31] => 246
            )
)

Im not sure if the above array structure is correct but you get the idea that I want to split the array into groups....