萌新求问这段代码啥意思啊

<?php
highlight_file('flag.php');
$_GET['id'] = urldecode($_GET['id']);
$flag = 'bdctf{xxxxxxxxxxxxxxxxxx}';
if (isset($_GET['user']) and isset($_POST['pass'])) {
if ($_GET['user'] == $_POST['pass'])
print 'pass can not be user.';
else if (sha1($_GET['user']) === sha1($_POST['pass'])&($_GET['id']=='margin'))
die('Flag: '.$flag);
else
print 'sorry!';
}
?>