PHP custom ordering array

Updated answer according to full code

The first issue here is that you're returning the result of uasort():

function sort_sizes($sizes)
{
    return uasort($sizes, array($this, 'cmp'));
}

That's wrong, because uasort() does not return the sorted array. It modifies the same variable that you pass as a parameter, and returns a boolean value. That's why you see 1 as output.

Make the method accept $sizes by reference:

function sort_sizes(array &$sizes)
{
    uasort($sizes, array($this, 'cmp'));
}

Then call it like so:

print_r($sizes);
$sorted_sizes = array_unique($sizes);
$this->sort_sizes($sorted_sizes);
print_r($sorted_sizes);

Here's your cmp() method, with added support for case-insensitive sorting:

function cmp($a, $b)
{
    $sizes = array('small' => 0, 'medium' => 1, 'large' => 2, 'xl' => 3, 'xxl' => 4);
    return $sizes[strtolower($a)] - $sizes[strtolower($b)];
}

Old answer

Try this. Use uasort() instead if you want to maintain key-value pairs:

function sort_sizes($a, $b) {
    // Map the sizes to an ordered sequence of ints
    static $sizes = array('small' => 0, 'medium' => 1, 'large' => 2, 'xl' => 3, 'xxl' => 4);

    // Find the difference, using the sizes as keys to the above array
    return $sizes[strtolower($a)] - $sizes[strtolower($b)];
}

$arr = array('Small', 'XXL', 'Medium', 'Large', 'XL');

print_r($arr); // Before sorting
uasort($arr, 'sort_sizes');
print_r($arr); // After sorting

Output:

Array
(
    [0] => Small
    [1] => XXL
    [2] => Medium
    [3] => Large
    [4] => XL
)
Array
(
    [0] => Small
    [2] => Medium
    [3] => Large
    [4] => XL
    [1] => XXL
)