注册页面用post传递，报Notice: Undefined index:

<?php

header('content-type:text/html;charset=utf-8');
$username=$password="";
echo var_dump(isset($username));
$username=$_POST['username'];
$password=$_POST['password'];
//$password1=$_POST['password1'];
//$email=$_POST['email'];
//$fav=$_POST['fav'];
//$verfy=$_POST['verify'];

$char=$username{0};
echo $char;
?>