Smoothing Data Array PHP

Question

I have an array:

Array
(
    [1] => 25
    [2] => 50
    [3] => 25
)

I would like to make it into:

Array
(
    [1] => 50
    [2] => 50
)

To do this I split the middle value between 1 and 3. This is the simplest example, where the split is 50,50. I would like to be able to take a 15 element array down to 6 elements.

Any ideas?

Additional Examples [10, 15, 20, 25] Reduced to two elements: 25(10 + 15),45(20 + 25) [10, 10, 10, 10, 11] Reduced to two elements: 25(10 + 10 + (10/2)),26((10/2) + 10 + 11)

Answer 1

After doing additional tests on Peter's solution, I noticed it did not get me what I expected if the reduce to size is an odd number. Here is the function I came up with. It also inflates data sets that are smaller then the requested size.

   <?php
        function reduceto($data,$r) {
            $c = count($data);

            // just enough data
            if ($c == $r) return $data;

            // not enough data
            if ($r > $c) {
                $x = ceil($r/$c);
                $temp = array();
                foreach ($data as $v) for($i = 0; $i < $x; $i++) $temp[] = $v;
                $data = $temp;
                $c = count($data);
            }

            // more data then needed
            if ($c > $r) {
                $temp = array();
                foreach ($data as $v) for($i = 0; $i < $r; $i++) $temp[] = $v;
                $data = array_map('array_sum',array_chunk($temp,$c));
            }
            foreach ($data as $k => $v) $data[$k] = $v / $r;
            return $data;
        }
    ?>