MySqli Commands out of sync; you can't run this command now

If you get Commands out of sync; you can't run this command now in your client code, you are calling client functions in the wrong order.

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without calling mysql_use_result() or mysql_store_result() in between.

From here: http://dev.mysql.com/doc/refman/5.0/en/commands-out-of-sync.html

Update

If you make the a variable for the query and paste the variable directly into something like MySQL Workbench you can check the syntax prior to execution.

<?php
            function myConnection(){
              $myConnection = mysqli_connect('localhost', 'my_user', 'my_password', 'my_db');
              return $myConnection;
            }   


    function register_user($register_data) { 
        array_walk($register_data, 'array_sanitize'); 
        //Make the array readable and seperate the fields from data 
        $fields = '`' . implode('`, `', array_keys($register_data)) . '`'; 
        $data = "'" . implode("', '", $register_data) . "'"; 
        //Insert the data and email an activation email to the user 
        $query = "INSERT INTO `members` ($fields) VALUES ($data)";
                    $myNewConnection = myConnection();          

                    if($result = mysqli_query($myNewConnection, $query)){ 
        email($register_data['mem_email'], 'Activate your account', "Hello " . $register_data['mem_first_name'] . ",nnThank you for creating an account with H Fencing. Please use the link below to activate your account so we can confirm you identity:nnhttp://blah.blah.co.uk/activate.php?mem_email=" . $register_data['mem_email'] . "&email_code=" . $register_data['email_code'] . "nn - David & Jay "); 
        mysqli_free_result($result);
         return ("Success");
        } else {
            echo $query;
            die(mysqli_error($myNewConnection));
        } 

    }

?>  

mysqli_connect("","username" ,"password","databasename");//Server name cannot be NULL

use loaclhost for server name(In Loacl)

<?php
    $con = mysqli_connect("localhost","username" ,"password","databasename");

    if (mysqli_connect_errno())
    {
        echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }
?>

Or can use MySQLi Procedural

<?php
    $servername = "localhost";
    $username = "username";
    $password = "password";

    // Create connection
    $con = mysqli_connect($servername, $username, $password);

    // Check connection
    if (!$con) {
        die("Connection failed: " . mysqli_connect_error());
    }
    echo "Connected successfully";
?>

EDIT 01

$servername = "localhost";
$username = "root";
$password = "";

There are result set pending from the query:

mysqli_multi_query($connection,$query);

You need to use/store result before you can proceed with next query after: Since you look like you don't really care about the first result set, do this after the multi query..

do
{
    $result = mysqli_store_result($connection);
    mysqli_free_result($result);
}while(mysqli_next_result());

Another alternative is to close the connection and starts it again..

mysqli_close($connection);
$connection = mysqli_connect("localhost","username","password","tbl_msgs");

It all depends on your requirements.