Merge sort in PHP - my first algorithmic exposure to PHP 5

I have this small program in PHP implementing merge sort. I used one rather simple run time optimisation via using one additional array which allows faster merge operations:

function merge_impl(&$source,
                    &$target,
                    $source_offset,
                    $target_offset,
                    $left_run_length,
                    $right_run_length) {
    $target_index = $target_offset;
    $left = $source_offset;
    $left_bound = $left + $left_run_length;
    $right = $left_bound;
    $right_bound = $right + $right_run_length;

    while ($left != $left_bound and $right != $right_bound) {
        $target[$target_index++] =
                $source[$right] < $source[$left] ?
                $source[$right++] :
                $source[$left++];
    }

    while ($left != $left_bound) {
        $target[$target_index++] = $source[$left++];
    }

    while ($right != $right_bound) {
        $target[$target_index++] = $source[$right++];
    }
}

function mergesort_impl(&$source,
                        &$target,
                        $source_offset,
                        $target_offset,
                        $range_length) {
    if ($range_length < 2) {
        return;
    }

    $middle = intval($range_length / 2);

    mergesort_impl($target,
                   $source,
                   $target_offset,
                   $source_offset,
                   $middle);

    mergesort_impl($target,
                   $source,
                   $target_offset + $middle,
                   $source_offset + $middle,
                   $range_length - $middle);

    merge_impl($source,
               $target,
               $source_offset,
               $target_offset,
               $middle,
               $range_length - $middle);
}

function mergesort_ex(&$arr, $from_index, $to_index) {
    $range_length = $to_index - $from_index;
    if ($range_length < 2) {
        return;
    }

    $aux = array_merge(array(), 
                       array_slice($arr, 
                                   $from_index, 
                                   $range_length));
    mergesort_impl($aux, $arr, 0, $from_index, $range_length);
}

function mergesort(&$arr) {
    mergesort_ex($arr, 0, count($arr));
}

$arr1 = array(23, 20, 4, -1, 5, 43, 22);
echo "The entire array is: ";

for ($i = 0; $i < count($arr1); ++$i) {
    echo $arr1[$i] . " ";
}

echo "<br>";

mergesort_ex($arr1, 1, 6);

echo "Sorted array slice A[1, 6]: ";

for ($i = 1; $i < 6; ++$i) {
    echo "$arr1[$i] ";
}

As I have almost no experience with PHP, please tell me anything that comes to mind (API, naming/coding conventions, etc.).