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.

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

更新

如果为查询创建变量并将变量直接粘贴到MysqL Workbench之类,则可以在执行之前检查语法.

<?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'],\n\nThank you for creating an account with H Fencing. Please use the link below to activate your account so we can confirm you identity:\n\nhttp://blah.blah.co.uk/activate.PHP?mem_email=" . $register_data['mem_email'] . "&email_code=" . $register_data['email_code'] . "\n\n - David & Jay "); 
        MysqLi_free_result($result);
         return ("Success");
        } else {
            echo $query;
            die(MysqLi_error($myNewConnection));
        } 

    }

?>