public function Check_UserName_Validate(){
    $CMD = "call UserName_Validate($this->input->post('edtUserName'));";
    $query = $this->db->query($CMD);
    if (mysqli_more_results($this->db->conn_id))
        mysqli_next_result($this->db->conn_id);
    return $query->row();
}

<div>
    First Name : <input type="text" id="edtFirstName" class="MyInputs">
    <br>
    Last Name : <input type="text" id="edtLastName" class="MyInputs">
    <br>
    Username : <input type="text" id="edtUserName" class="MyInputs">
    <br>
    Password : <input type="password" id="edtPassword" class="MyInputs">
</div>

public function Signup($Flag = 0){
...
    $data["ErrorMSG"] = "";
    $data["ErrorKind"] = 0;
    $this->form_validation->set_rules("edtFirstName", $data["FirstName_Caption"], "trim|required");
    $this->form_validation->set_rules("edtLastName", $data["LastName_Caption"], "trim|required");
    $this->form_validation->set_rules("edtUserName", $data["Username_Caption"], "trim|required");
    $this->form_validation->set_rules("edtPassword", $data["Password_Caption"], "trim|required");
    if ($Flag == 1){
        if ($this->form_validation->run() == FALSE) {
            $data["ErrorMSG"] = validation_errors();
            $data["ErrorKind"] = 4;
            $this->load->view("Signup", $data);
        }
        else{
            /* Insert user information to database */
        }
    }
    else{
        $this->load->view("Signup", $data);
    }
}