PHP问题$ _GET

$_GETvar 有一个非常奇怪的问题.

这是我的代码:

if(isset($_GET['offer'])) {
    $params = array();
    $params['wifi'] = "%wifi%";
    $params['publisher'] = "%pubid%";
    $params['framework'] = "%framework%";
    $params['date'] = "%cachebuster%";

    foreach($params as $key=>$param){

    $array[$_GET[$key]] = $_GET[$key];

    }

    print_r($array);
}

这是我发送的网址:http: //example.com/track/？offer = g58fFPK49fk4&click_id =%guid%&wifi =%wifi%&publisher = appubid%&framework =%framework%&date =%cachebuster%

这是我得到的奇怪输出:

Array ( [%wifi%] => %wifi% [%pubid%] => %pubid% [%framework%] => %framework% [Êchebuster%] => Êchebuster% )

为什么%cachebuster%会变成Êchebuster%？