Undefined variable

<?php 
$Wanted_Username = "hier loginnaam"; 
$Wanted_Password = "hier wachtwoord"; 

if($username) 
{ 
if(!$password) 
{ 
die("Enter a password<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
if($username != $Wanted_Username) 
{ 
die("Incorrect Details<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
if($password != $Wanted_Password) 
{ 
die("Incorrect Details<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
} 
if(!$username) 
{ 
?> 
<form method="post"> 
<table> 
<tr><td colspan=2><b><font color=red>enter in your username and password</font></b></td></tr> 
<tr><td>username</td><td><input type=text name=username maxlength=150></td></tr> 
<tr><td>password</td><td><input type=password name=password></td></tr> 
<tr><td colspan=2><input type=submit value="login"></td></tr> 
</table> 
</form> 
<?php 
die(""); 
} 
?>

<?php 
$Wanted_Username = "hier loginnaam"; 
$Wanted_Password = "hier wachtwoord"; 
 
if($username) 
{ 
if(!$password) 
{ 
die("Enter a password<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
if($username != $Wanted_Username) 
{ 
die("Incorrect Details<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
if($password != $Wanted_Password) 
{ 
die("Incorrect Details<br><a href=\"javascript:history.go(-1)\">Back</a>"); 
} 
} 
if(!$username) 
{ 
?> 
<form method="post"> 
<table> 
<tr><td colspan=2><b><font color=red>enter in your username and password</font></b></td></tr> 
<tr><td>username</td><td><input type=text name=username maxlength=150></td></tr> 
<tr><td>password</td><td><input type=password name=password></td></tr> 
<tr><td colspan=2><input type=submit value="login"></td></tr> 
</table> 
</form> 
<?php 
die(""); 
} 
?>