PHP md5 Hakkında Köprü için bir soru ?

<? 
function forum_registeruser($username_in, $password_in, $email_in, $usergroup_in){

$connect = mysql_connect("localhost","USER","Şifre") or die ("Could not even connect to the database");//kendinize göre şifrelerinizi girin..

mysql_select_db("mybb");//forum database..

$query="SELECT username FROM users WHERE username='".$username_in."'"; //tablonuzu girin...

$checkexistence = mysql_query($query) or die("Could not read from table");

$exists=false;

while ($rows=mysql_fetch_array($checkexistence)){
extract($rows);
if($username==NULL){
    $exists=false;
}else{
    $exists=true;
}
}

if($exists==false){
    $salt=random_str('8');
    $password_hash=salt_password($password_in, $salt);
    $login_key=generate_loginkey();
        
    $insert_query = "INSERT INTO users (username, password, salt, loginkey, email, usergroup) VALUES ('".$username_in."', '".$password_hash."', '".$salt."', '".$login_key."', '".$email_in."', '".$usergroup_in."')";
    $insertuser = mysql_query($insert_query) or die("Could not insert data into table");

echo('Kullanıcı Başarılı Olarak Kaydedilmiştir..');
        
    }else{
        echo('Bu Kullanıcı adı sisteme kayıtlı...');
    }
    
}

function random_str($length="8")
{
    $set = array("a","A","b","B","c","C","d","D","e","E","f","F","g","G","h","H","i","I","j","J","k","K","l","L","m","M","n","N","o","O","p","P","q","Q","r","R","s","S","t","T","u","U","v","V","w","W","x","X","y","Y","z","Z","1","2","3","4","5","6","7","8","9");
    $str = '';

    for($i = 1; $i <= $length; ++$i)
    {
        $ch = mt_rand(0, count($set)-1);
        $str .= $set[$ch];
    }

    return $str;
}

function salt_password($password, $salt)
{
    return md5(md5($salt).md5($password));
}

function generate_loginkey()
{
    return random_str(50);
} 
$mail = "osman8@hotmail.com";
forum_registeruser(osmanyxss, 12345678, $mail, 2);
?>