函数
isset
检测变量是否已设置并且非 NULL
parse_url
解析 URL，返回其组成部分
$url = 'http://username:password@hostname/path?arg=value#anchor';
print_r(parse_url($url));
输出结果：
Array
(
[scheme] => http
[host] => hostname
[user] => username
[pass] => password
[path] => /path
[query] => arg=value
[fragment] => anchor
)
