Create simple PHP&MYSQL message board from classic

<?php
require_once('');

$username = $_POST['name'];
$info = $_POST['info'];
$id = $_GET['id'];
//echo $id;

$sql = "update lo set username= '".$username."',info='".$info."' where <script>alert('Modification was successful!');='';</script>";
?>

<?php
session_start();
require_once('');

$id=$_GET['id'];

if($_SESSION["key"]==1)
{
$sql = "delete from lo where <script>=''</script>";
}
else
{
header('location:');
}
?>

<?php
require_once('');
require_once('');
?>

<div >
    <form method="post" action="">
<h1>Administrator login</h1>
<p>Name: <input type="text" name="name" size="20" class="y" />&nbsp;</p>
Password: <input type="password" name="password" size="20" class="y" //&nbsp;</p>
<p class="cen"><input type="submit" value="administrator login"//</p>
<p class="cen1">Silver Message Board Version 1.0</p>
    </form>
</div>

<?php
session_start();
require_once('');

$name = $_POST['name'];
$password = $_POST['password'];

$sql = "select * from admin where name='".$name."'";
$result = mysql_query($sql);
$num = mysql_num_rows($result);

if (strrpos($name,"<")!==false || strrpos($name,">")!==false||strrpos($name,"@")!==false||strrpos($name,"\"")!==false||strrpos($name,"'")!==false||strrpos($name,"_")!==false)
{
echo "<script>alert('can't have special characters!');='';</script>";
}

if($num)
{//If the user exists, check whether the password is correct
    $rs = mysql_fetch_array($result);
    if($rs[2]!=$password)
    {
echo "<script>alert('Password is incorrect, please confirm and enter it!');='';</script>";
    }
    else
{//The username and password are correct, register the SESSION variable, and then jump to the homepage
        $_SESSION["key"]=1;
echo "<script>alert('Login successfully!');='';</script>";
    }
}
else
{//If there is no such user
echo "<script>alert('There is no such user, please confirm and enter it!');='';</script>";
}
?>

<?php
session_start();
$_SESSION["key"] = 0;//Make SESSION not 1, 0 is a tourist, 1 is an administrator

header('location:');
?>