PHP - 在变量中生成并将xml作为字符串存储

I'm trying to store a xml as string in a variable so that I can store it in my database.

$xml = "<root>";
foreach(...){
    $xml .= "<user id='$id'/>";
}
$xml .= "</root>";

When I echo it, it's not displayed at all as if my web brower reads it as html tag. It doesn't even look like $xml is storing those as texts. Now, I'm trying to do it with DOMDocument... not not quite successful yet. Any tips? :(

• Edited my stupid += mistakes..

PHP uses a . as a concatenate operator, or a .= as a shortcut, not a + or +=.

$xml = "<root>";
foreach(...){
    $xml .= "<user id='$id'/>";
}
$xml .= "</root>";