Check.php

<?php
$host="localhost"; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name=""; // Database name
$tbl_name=""; // Table name

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");

// username and password sent from form
$myusername=$_POST['username'];
$mypassword=$_POST['passwort'];

// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes($myusername);
$mypassword = stripslashes($mypassword);
$myusername = mysql_real_escape_string($myusername);
$mypassword = mysql_real_escape_string($mypassword);

$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){
$r = mysql_fetch_assoc($result);    
// Register $myusername, $mypassword and redirect to file "login_success.php"
$_SESSION['myusername'] = $r['username'];
$_SESSION['mypassword'] = $r['password'];
header('Location: /play.php?id='.$r['id']);
}
else {
echo "<b>Falscher Username oder Passwort</b>";
}
?>

mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
...
$result=mysql_query($sql);

$link = mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name", $link)or die("cannot select DB");
...
$result=mysql_query($sql, $link);

$result=mysql_query($sql);

print_r ($result); exit;