我从一个网站获得了帮助，并使用了他们的大部分代码。我理解了代码，但无法找出错误。
//main_login.php
<html>
<title>User Login Form</title>
<head></head>
<body>
<form name="form1" method="post" action="newcheckLogin.php">
<td>
<table width="100%" border="0" cellpadding="3" cellspacing="1" bgcolor="#FFFFFF">
<tr>
<td width="80">Username</td>
<td width="6">:</td>
<td width="300"><input name="myusername" type="text" id="myusername"></td>
</tr>
<tr>
<td>Password</td>
<td width="6">:</td>
<td><input name="mypassword" type="password" id="mypassword"></td>
</tr>
<tr>
<td> </td>
<td> </td>
<td><input type="submit" name="Submit" value="Login"></td>
</tr>
</table>
</td>
</form>
//newchecklogin.php- 这用于检查用户和查询数据库。
<?php
include("config.php");
session_start();
#echo "Check Login\n";
?>
<html>
<head><title></title>
<form name="form2" method="post" action="Logout.php">
<p align="right">
<input type="submit" name="Logout" id=Logout" >
</p>
</form>
</head>
</html>
<?php
$myusername=stripslashes(mysql_real_escape_string($_POST['myusername']));
$mypassword=md5(stripslashes(mysql_real_escape_string($_POST['mypassword'])));
$sql="select jobid from jobs where userID='$myusername'";
echo "Hey ".$myusername." ";
$result=mysql_query($sql) or die(mysql_error());
$row=mysql_fetch_array($result);
$active=$row['active'];
if(mysql_num_rows($result)==1){
echo "Username and password found";
session_register("myusername");
$_SESSION['login_user']=$myusername;
header("location:welcome.php");
}
else{
$error="Your login Name or Password is invalid";
}
?>
//欢迎.php
<?php
include('lock.php');
include('newchecklogin.php');
?>
<body>
<h1>Hello <?php echo $login_session; ?></h1>
<?php
echo "<br>You have ".$mysql_num_rows($result)." jobs<br>";
echo "<table border=1 cellpadding=4><th>Job id</th><br>";
while ( $r = mysql_fetch_assoc($result) ) {
echo "<tr><td>";
echo $r['jobid']."</td></tr>";
}
echo "</table>";
?>
</body>
//lock.php
<?php
include('config.php');
session_start();
$user_check=$_Session['login_user'];
$ses_sql=mysql_query("select userID from jobs where userID='$user_check'");
$row=mysql_fetch_array($ses_sql);
$login_session=$row['userID'];
if(!isset($login_session)){
header("Location:login.php");
}
?>
请帮忙，如果有人能再次向我解释如何设置会话，那就太好了。