Sorting Product Collection – Sort by SKU Array

I have an array that contains some SKUs belonging to the current category.

I want to reorder the collection based on that array so that the SKUs present in my order array will come first.

For this i have been trying to edit the list.phtml file

$category = Mage::registry('current_category');
$inArray = explode(',', $category->getData('top_skus'));

usort($_productCollection, function($a, $b) use ($inArray){ 
    print_r($a);
    $aAge = $a['sku'];
    $bAge = $b['sku'];
    $aWeight = 0;
    $bWeight = 0;
    if (in_array($aAge, $inArray)) 
        $aWeight++;

    if (in_array($bAge, $inArray)) 
        $bWeight++;

    if ($aWeight != $bWeight) {
        return $aWeight > $bWeight ? -1 : 1;
    } else if ($aWeight > 0) {
        // need to sort by order which specified in array
        $aIndex = array_search($aAge, $inArray);
        $bIndex = array_search($bAge, $inArray);
        return ($aIndex == $bIndex ? 0 : ($aIndex > $bIndex ? 1 : -1));
    } else {
        // just compare age values
        return ($aAge == $bAge ? 0 : ($aAge > $bAge ? 1 : -1));
    }
});

I am getting an error like this

Warning: usort() expects parameter 1 to be array, object given  in C:\wamp\www\magento1\app\design\frontend\base\default\template\catalog\product\list.phtml on line 74

How to supply the array that contains the products to my usort? From where i will get that array?