在作业之前是否未设置？

class RouteCollection implements \IteratorAggregate, \Countable
{
    /**
     * @var Route[]
     */
    private $routes = array();

    public function add($name, Route $route)
    {
        unset($this->routes[$name]);

        $this->routes[$name] = $route;
    }

     public function remove($name)
     {
        foreach ((array) $name as $n) {
            unset($this->routes[$n]);
        }
     }
}

This is a piece of code from the class Symfony\Component\Routing\RouteCollection. Does unset before the assignment matter?

Why is it done?

Second question: Why in remove method simple string is parsed to array?

Why I can't use simply:

unset($this->routes[$name]);

Same as in add method?

Does unset before the assignment matter?

It can. If $this->routes[$name] is a PHP reference and you don't use unset, all symbols pointing to the underlying value will point to the new value. If you use unset before, the assignment will only affect the symbol used.

Second question: Why in remove method simple string is parsed to array?

This is just a small trick that allows you to use the remove method with both strings and array. When you cast a string (e.g. 'abc') to an array, PHP will return an array that contains the string as its only value (array(0 => 'abc')). For strings the method works as you suggested, and for arrays it will unset all names in the array.