mysqli :: prepare（）vs mysqli_stmt :: prepare（）

Question

I was using the following code to make a query with prepared statements: I used mysqli::prepare(), NOT mysqli_stmt::prepare(), so $res does not have to be initialized. It is returned by mysqli::prepared. php.net link

$sql2 = "INSERT INTO best_scores(`user_id`, `test_id`, `score`) VALUES(?, ?, ?)";
if($res = $db->prepare($sql2) == FALSE) echo $db->error . "
";
$res->bind_param("sii", $user_id, $_GET['test'], $max);
$res->execute();
$res->free_result();

When that code was reached, the following error appeared:

Call to a member function bind_param() on a non-object

I looked for answers but all the questions had mistakes in the SQL query string or they didn't free the previous results. None of them helped me, so I tried to change something. I changed my code into this:

$sql2 = "INSERT INTO best_scores(`user_id`, `test_id`, `score`) VALUES(?, ?, ?)";
$res = $db->stmt_init();
if($res->prepare($sql2) == FALSE) echo $db->error . "
";
$res->bind_param("sii", $user_id, $_GET['test'], $max);
$res->execute();
$res->free_result();

Now it is working. But why is it different? What was the problem with the first attempt?

Edit: My $db object is defined like this:

$db = new mysqli("localhost", "user", "pass", "database");
if($db->connect_errno > 0) {
    die('Unable to connect to database [' . $db->connect_error . ']');
}

Answer 1

As we discuss it in the comments, it might be a wrong assignation in the if() block.

What do I tested. If block, like yours:

$x = (object)array('a'=>'b');

if ($a = $x == false) {
    echo 'a';
}

var_dump($a);

Output:

boolean false

Then I have put parentheses around the assignation:

$x = (object)array('a'=>'b');


if (($a = $x) == false) {
    echo 'a';
}

var_dump($a);

Output:

object(stdClass)[1] public 'a' => string 'b' (length=1)

so you would need to put parentheses around the assignation for $res

if(($res = $db->prepare($sql2)) == FALSE) 

Because your way, you wrongly reassign $res to FALSE and the object has been lost