Why does this usort()-function fail on some versions of PHP?

Question

We wrote some code involving usort which works fine on our development systems (PHP 5.2.8), but are experiencing a problem on our live systems (PHP 5.2.0):

// Sort by distance
usort($locations, 'Interpolator::sortByDistance');

calls the method (within the same class Interpolator):

private static function sortByDistance($a, $b) {
    $return = 0;
    if($a['distance'] > $b['distance']) {
        $return = 1;
    } else if ($a['distance'] < $b['distance']) {
        $return = -1;
    }
    return $return;
}        

On our live systems, this returns a completely arbitrarily sorted array, the original order is disturbed, but still not sorted by distance.

I cannot find any reference to a PHP bug fixed between 5.2.0 and 5.2.8 relevant to this problem.

Where might this problem be coming from? Can I fix this short of writing a sorting function myself?

Answer 1

The only think I can think of is that you should be using this:

usort($locations, array('Interpolator', 'sortByDistance'));