An associative array with known keys can be easily custom sorted using a switch statement in the callback:

NB ksort can be used beforehand to ensure expected results

ksort($array);

uksort($array, function ($a) {                    
    switch($a) {
        case 'pepperoni':
            return 0;
        case 'beef':
            return 1;
        case 'chicken':
            return 2;
        case 'ham':
            return 3;
        case 'vegetarian':
            return 4;
    }
});