I came up with this. Tested it and confirmed it works. Reordered your array so I could actually see the sorting.
$arr = Array(
1 => "Fried_Chicken",
2 => "Donuts",
3 => "Bananas",
4 => "Apples",
5 => "Eclairs",
6 => "Cherries"
);
// Get donut and key
$donut_key = array_search("Donuts", $arr);
$donut = $arr[$donut_key]; // If you don't need to keep the value, skip this line
// Remove donut
unset($arr[$donut_key]);
// Sort
asort($arr);
// Append Donut
$arr += array($donut_key => $donut);
Array Search http://php.net/manual/en/function.array-search.php
Key preserving append http://www.vancelucas.com/blog/php-array_merge-preserving-numeric-keys/