如何在将字段提交到其他文件之前验证字段并在同一页面中显示错误消息，例如：*必填字段
<?php
// Inialize session
session_start();
// Check, if user is already login, then jump to secured page
if (isset($_SESSION['username'])) {
header('Location: Pro_Lesson.php');
}
?>
<html>
<head>
<title>PHPMySimpleLogin 0.3</title>
</head>
<body>
<h3>User Login</h3>
<table border="0">
<form method="POST" action="log.php">
<tr><td>Username</td><td>:</td><td><input type="text" name="username" size="20"></td></tr>
<tr><td>Password</td><td>:</td><td><input type="password" name="password" size="20"></td></tr>
<tr><td> </td><td> </td><td><input type="submit" value="Login"></td></tr>
</form>
</table>
</body>
</html>