注意：未定义的变量：[value] [duplicate]

I have searched on stackoverflow about the undifined variable, and I couldn't find anything that solved my problem.

this is my PHP code:

if(isset($_POST['login'])){

    $username =mysql_real_escape_string( $_POST['naam']);
    $password =md5( $_POST['wacht']);

    $check_user = "select * from users where gebruikersnaam='$gebruikersnaam' AND wachtwoord='$wachtwoord'";

    $run =mysql_query($check_user);

    if(mysql_num_rows($run)>0){

    $_SESSION['naam']=$gebruikersnaam;

        echo "<script>window.open('welkom.php','_self')</script>";
    }

This is my HTML code:

<div id=content>
    <form method='POST' action='login.php'>
        Gebruikersnaam:<br>
        <input type='text' name='naam' />
        <br>
        <br>
        Wachtwoord:<br>
        <input type='password' name='wacht' />
        <br>
        <br>
        <input type='submit' name='login' value='Inloggen' />
    </form>
    <br>
    Niet geregistreerd? <a href='registratie.php'>Klik hier.</a>
</div>  

I can't see where my problem is, it says I have a undefined variable on this line: $check_user = "select * from users where gebruikersnaam='$gebruikersnaam' AND wachtwoord='$wachtwoord'";

The names I use are from my database so I wouldn't make a mistake with the names.

Could you people help me? It would be a real time saver because I can't find the problem.

</div>

You do not check if the following variables are set before you assign them.

$_POST['naam']
$_POST['wacht']

As you did with $_POST['login'] you can use isset to check they exist before assignment.

if (isset($_POST['naam'] && isset($_POST['wacht']) {
    // Do something...
}

In addition to this in the query you are using the variables $gebruikersnaam and $wachtwoord which you don't appear to be referencing anywhere else? So after some google translating I'm guessing that you intended for this bit of code:

$username =mysql_real_escape_string( $_POST['naam']);
$password =md5( $_POST['wacht']);

To be the following:

$gebruikersnaam = mysql_real_escape_string($_POST['naam']);
$wachtwoord     = md5($_POST['wacht']);

Hopefully that helps, just a bit of a side note though, I would really advise reading over http://www.phptherightway.com/ and getting familiar with some of the best practices for PHP.

In your code I would attempt to refactor it and utilise password_hash() and mysqli_* as MD5() is not secure and the mysql_ extension has been removed in the latest version of PHP and was deprecated before that.

Its just a typo I think, you create 2 variables called $username and $password but you are not using them in the query, the query is using $gebruikersnaam and $wachtwoord, which of course have not been defined.

So amend the query to this

if(isset($_POST['login'])){

    $username =mysql_real_escape_string( $_POST['naam']);
    $password =md5( $_POST['wacht']);

    $check_user = "SELECT * 
                   FROM users 
                   WHERE gebruikersnaam='$username' 
                     AND wachtwoord='$password'";

    $run =mysql_query($check_user);

    if(mysql_num_rows($run)>0){

    $_SESSION['naam']=$gebruikersnaam;

        echo "<script>window.open('welkom.php','_self')</script>";
}

Additional Note:

Please dont use the mysql_ database extension, it is deprecated (gone for ever in PHP7) Especially if you are just learning PHP, spend your energies learning the PDO or mysqli_ database extensions, and here is some help to decide which to use