看代码，找问题

[panic2]

以下验证用户admin abc123的代码通不过，总要我输密码：
<?php
if (!isset($PHP_AUTH_USER)) {
header('WWW-Authenticate: Basic realm="My Private Stuff"');
header('HTTP/1.0 401 Unauthorized');
echo 'Authorization Required.';
exit;
}
else if (isset($PHP_AUTH_USER)) {
if (($PHP_AUTH_USER != "admin") || ($PHP_AUTH_PW != "abc123")) {
header('WWW-Authenticate: Basic realm="My Private Stuff"');
header('HTTP/1.0 401 Unauthorized');
echo 'Authorization Required.';
exit;
} else {
echo "<P>You're authorized!</p>";
}
}
?>

[panic2]

我知道了：
$_server["http_auth_user"]