Simple Login Passalt7

<?php
error_reporting();
session_start();

$password = "ec:550dfdef89bed03f550dfdef89bed03f"; // 123

function login($key,$password){
$jum_key = strlen($key);
$enc = md5($key);
$jum_key = substr(md5($jum_key),0,2);
$enc_1 = substr($enc, 18);
$enc_2 = substr($enc, 0,17);
$hasil = "$jum_key:".substr(md5($enc_1),0,16).substr(md5($enc_1),0,16);
if($hasil==$password){
    return true;
}else{
    return false;
}
}

if($_POST['Passalt7']){
    $cek = login($_POST['password'],$password);
    if($cek){
        $_SESSION["login"]=$_POST['password'];
        header('Location: ?');
    }else{
        echo "gagal";
    }
}

if(!isset($_SESSION["login"])){
?>
<!DOCTYPE html>
<html>
<head>
    <title>Login Page</title>
</head>
<body>
<form action="" method="post">
    <input type="text" name="password">
    <input type="submit" name="Passalt7">
</form>
</body>
</html>
<?php
}else{
    echo "HALOOO";
}
?>