PHP file_exists returns false with existing files

I'm having a little problem with my PHP-code. I want to create a site which has different directories. Every directories has a file named pass (not .txt or something) with the value of the directories password. So if the pass-file doesn't exist the group will not exist. But even if the group exists it still says the group doesn't exist. Here's my code:

<?php
$name = $_POST['name'];
$group = $_POST['group'];
$pass = $_POST['pass'];
$filename = '/groups/' . $group . '/pass';
if(file_exists($filename)){
    $handle = fopen($filename) or die("can't open file");
    $hoi = fread($handle, filesize($filename));
    fclose($handle);
    if ($pass === $hoi){
        session_start();
        $_SESSION['name'] = $name;
        header('Location: http://www.google.com');
    }
    else{
        echo 'Password is wrong!';
    }
}
else{
    echo 'Group does not exist!';
}
?>

All POST-data is correct btw. Thanks for your help!