Shuffle the order of keys in an associative array, if they have the same values?

Given an associative array like this, how can you shuffle the order of keys that have the same value?

array(a => 1,
      b => 2,  // make b or c ordered first, randomly
      c => 2,
      d => 4,
      e => 5,  // make e or f ordered first, randomly
      f => 5);

The approach I tried was to turn it into a structure like this and shuffle the values (which are arrays of the original keys) and then flatten it back into the original form. Is there a simpler or cleaner approach? (I'm not worried about efficiency, this is for small data sets.)

array(1 => [a],
      2 => [b, c],  // shuffle these
      4 => [d], 
      5 => [e, f]); // shuffle these


function array_sort_randomize_equal_values($array) {
    $collect_by_value = array();
    foreach ($array as $key => $value) {
        if (! array_key_exists($value, $collect_by_value)) {
            $collect_by_value[$value] = array();
        }
        // note the &, we want to modify the array, not get a copy
        $subarray = &$collect_by_value[$value];
        array_push($subarray, $key);
    }

    arsort($collect_by_value);

    $reordered = array();
    foreach ($collect_by_value as $value => $array_of_keys) {
        // after randomizing keys with the same value, create a new array
        shuffle($array_of_keys);
        foreach ($array_of_keys as $key) {
            array_push($reordered, $value);
        }
    }

    return $reordered;
}