PHP if Abfrage will nicht. Falsch definierte Variable?

(...)
<div id="content">  
<h1>Log In:</h1>
Hallo!<br><br>
<form method="post" action="LogIn.php">  
Username:<br>
 <input type="text" name="username-" cols="20" maxlength="20" style="height:18px;"><br>Passwort<br>  
 <input type="password" name="pw" cols="20" maxlength="20" style="height:18px;"><br>  
<input type="submit" id="btn" onclick="login()" name="login" value="Log In" >  
</form>

<br>

<?php
$data = '03.txt';   
$file = file($data); 
$a2 = $file;
$b2 = $file[1];
ob_start();
echo "  ".$a2."  ".$b2;  
if (isset($_POST['login'])) {  
$a = $_POST['username-'];  
$b = $_POST['pw'];  
echo $b.$a."<br> <br> <br>";  
if ($a == $a2 && $b == $b2) {
ob_clean();
ob_start();
	echo "Herzlich willkommen ".$a;  
}
else {
ob_clean();
ob_start();
echo "Log in leider Fehlgeschlagen.<br> ";  
echo "<br>angemeldet mit:<br>Username: ".$a." <br>Passwort: ".$b;  
}
}

?>
(...)