$addresses = "'email1@yahoo.com','email2@gmail.com'";
$to = array($addresses);

不会神奇地创建一个包含两个元素的数组.您将获得的是数组中的单个元素，例如

is not going to magically create an array with two elements in it. What you'll get is a SINGLE element in the array, e.g

$to = array(
    0 => "'email1@yahoo.com','email2@gmail.com'"
);

但是，做

$to = explode(',', $addresses);

将为您提供两个元素的数组:

WILL give you a two element array:

$to = array (
    0 => "...yahoo",
    1 => "...gmail"
);