php提交按钮点击后没反应，各位高手帮帮忙
这是源代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
</head>
<body>
<table width="283" height="85" border="1" cellpadding="0" cellspacing="0">
<form name="form1" method="post" action="test2.php">
<tr>
<td width="114" height="30" align="center" class="style1">管理员:</td>
<td width="163" height="30" align="center"><input name="username" type="text" id="username4" size="20"></td>
</tr>
<tr>
<td> </td>
<td height="25"> <input type="submit" name="Submit" value="提交"></td>
</tr>
</form>
</table>
<?php
if($Submit=="提交"){
$username=$_POST[username];
}
?>
<table width="284" boder="1" cellpadding="0" cellspacing="0">
<tr>
<td height="25" align="center" class="style1"><?php echo"管理员：$username"?></td>
</tr>
</body>
</html>
