Потупчик - вставка переменных в запрос

<?php
session_start();
$reg_info = array(
    'name' => $_POST['name'],
    'e_mail' => $_POST['e_mail'],
    'password' => $_POST['password'],
);
$name = $reg_info['name'];
$e_mail = $reg_info['e_mail'];
$password = $reg_info['password'];
$reg_db = mysqli_connect('localhost', 'db', 'password');
mysqli_select_db($reg_db, 'db');
$isSet_user_tmp = mysqli_query($reg_db, 'SELECT * FROM users_db WHERE user_name="$name"');
$isSet_user = mysqli_fetch_array($isSet_user_tmp, MYSQLI_ASSOC);
echo "<pre>";
print_r(mysqli_error($reg_db));
//print_r($isSet_user_tmp);
if($isSet_user['name']){
    echo "this name is reserved";
}elseif($isSet_user['e_mail']){
    echo "this eMail id reserved";
}else{
    $reg_user = mysqli_query($reg_db, 'INSERT INTO users_db (user_name, e_mail, sequrity, path) VALUES ("$name", "$e_mail", "$password", "$name")');
}
$tmp = mysqli_query($reg_db, 'SELECT * FROM users_db');
$result = mysqli_fetch_array($tmp);
print_r ($result);
mysqli_close($reg_db);
?>

$reg_user = mysqli_query($reg_db, "INSERT INTO `users_db` (`user_name`, `e_mail`, `sequrity`, `path`) VALUES ('{$name}', '{$e_mail}', '{$password}', '{$name}')");

$var = isset($_GET['var']) ? mysqli_real_escape_string($reg_db, trim($_GET['var'])) : '';

mysqli_query($reg_db, 'INSERT INTO `users_db` (`user_name`, `e_mail`, `sequrity`, `path`) VALUES ("{$name}", "{$e_mail}", "{$password}", "{$name}")');

mysqli_query($reg_db, 'INSERT INTO `users_db` (`user_name`, `e_mail`, `sequrity`, `path`) VALUES ("{$name}", "{$e_mail}", "{$password}", "{$name}")');

mysqli_query($reg_db, "INSERT INTO `users_db` (`user_name`, `e_mail`, `sequrity`, `path`) VALUES ('{$name}', '{$e_mail}', '{$password}', '{$name}')");

$stmt = mysqli_prepare($reg_db, 'INSERT INTO `users_db` (`user_name`, `e_mail`, `sequrity`, `path`) VALUES (?, ?, ?, ?)');
mysqli_stmt_bind_param($stmt, 'ssss', $name, $e_mail, $password, $name);
mysqli_stmt_execute($stmt);
mysqli_stmt_close($stmt);

$a = '123';
echo "$a"; // выведет  123
echo '$a'; // выведет  $a