在PHP/Javascript中向前获取301/302的源url

<?php
$url = 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
$headers = get_headers($url, 1);
if (isset($headers['Location'])) {
    $sourceUrl = $headers['Location'];
    echo '源URL：' . $sourceUrl;
} else {
    echo '没有重定向';
}
?>

var xhr = new XMLHttpRequest();
xhr.open('GET', window.location.href, true);
xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status >= 300 && xhr.status < 400) {
        var sourceUrl = xhr.getResponseHeader('Location');
        console.log('源URL：' + sourceUrl);
    } else {
        console.log('没有重定向');
    }
};
xhr.send();