Object Oriented and Procedural Mysqli Help?

I asked a question earlier and a user stated that it looks like i'm oddly mixing the OO and procedural style usage of mysqli and that I should stick to either one or the other.

Can some one show me exactly what is wron开发者_高级运维g with my code and how my code should look like in OO and procedural form. I'm kind of curious now since the code I have seems to work for me, but then again I'm fairly new to PHP and MySQL and would like to learn the correct way.

Here is the code.

// Query member data from the database and ready it for display
$mysqli = new mysqli("localhost", "root", "", "sitename");
$dbc = mysqli_query($mysqli,"SELECT users.*, categories.*, users_categories.* FROM users_categories INNER JOIN users ON users_categories.user_id = users.user_id INNER JOIN categories ON users_categories.category_id = categories.id WHERE users_categories.user_id=3");

if (!$dbc) {
    // There was an error...do something about it here...
    print mysqli_error($mysqli);
}

// Query member data from the database and ready it for display
$mysqli = new mysqli("localhost", "root", "", "sitename");
$dbc = $mysqli->query("SELECT users.*, categories.*, users_categories.* FROM     users_categories INNER JOIN users ON users_categories.user_id = users.user_id INNER JOIN categories ON users_categories.category_id = categories.id WHERE users_categories.user_id=3");

if (!$dbc) {
    // There was an error...do something about it here...
    print $mysqli->error();
}

And I don't think you should start learning oop this way. I recommend you http://php.net/manual/en/oop5.intro.php

I have used this in the past don't know if it helps, as this is not really the answer to your problem, but might be useful.

    <?php // cnxVars
    class cnxVars {

 private $host = "localhost";
 private $host_un = "username";
 private $host_pass = "test123";
 private $host_db = "si";

 public function cnx() { // Connect to MySQL
  $this->cnx = mysql_connect($this->host, $this->host_un, $this->host_pass) or mysql_error(); }

 public function db() { // Select DB
  return mysql_select_db($this->host_db, $this->cnx) or mysql_error(); }


    } // End cnxVars ?>

then you can do this

$mysql = new cnxVars();
$mysql->cnx();
$mysql->db();