如何从PHP codeigniter中的字段中减去一个值？

i am have a table Reseller and Users. Now Each user is mapped by his Reseller with a field called KEY which uniquely defines the reseller! now I have a form where i edit a user. in here if reseller sets the balance of user to yes then i want to subtract 5 euros from the balance field of Reseller. Below is my code:

the controller to check if reseller has set balance to yes:

 if($this->input->post('balance') === "Yes")
         {
            $this->reseller_m->deduct();
         }

the Mode function which should subtract 5 from reseller balance:

public function deduct($balance)
{
    $this->db->set('balance', 'balance-5');
    $this->db->where('id' , $id);
    $this->db->update('reseller',$data);
}

the edit code :

public function edit ($id = NULL)
{

     $usertype=$this->session->userdata('usertype');
    if($usertype ==="reseller")
    {

    // Fetch a user or set a new one
    if ($id) {
        $this->data['user'] = $this->user_m->get($id);
        count($this->data['user']) || $this->data['errors'][] = 'User could not be found';
    }
    else {
        $this->data['user'] = $this->user_m->get_new();
    }

    // Set up the form
    $rules = $this->user_m->rules_admin;
    $id || $rules['password']['rules'] .= '|required';
    $this->form_validation->set_rules($rules);

    // Process the form
    if ($this->form_validation->run() == TRUE) {
        $data = $this->user_m->array_from_post(array('sip_id','sip_pass','name','email', 'password','phone','status','created','balance'));
        $data['password'] = $this->user_m->hash($data['password']);



        $this->user_m->save($data, $id);


         if($this->input->post('balance') === "Yes")
         {

            $this->reseller_m->deduct();

            //echo "goimng";
         }


        redirect('reseller/user');
    }

    // Load the view
    $this->data['subview'] = 'reseller/user/edit';
    $this->load->view('reseller/_layout_main', $this->data);


}
else{
    $this->load->view('permission');
}

}

help me to subtract 5 from reseller only when he edits his own user and should not affect others.

Try this in your model, where $id is the id of the reseller.

public function deduct($id)
{
    $this->db->set('balance', 'balance-5', false);
    $this->db->where('id' , $id);
    $this->db->update('reseller');
}

Alse, in your controller you must know the reseller's id, and use it when you call the model function:

 if($this->input->post('balance') === "Yes")
         {
            $this->reseller_m->deduct($id); //you must have the $id (reseller's id) value set
         }

Try $this->db->set('balance', 'balance-5', FALSE);

set() will also accept an optional third parameter ($escape), that will prevent data from being escaped if set to FALSE.

Use FALSE as third parameter in your set

$this->db->set('balance', 'balance-5',FALSE);

Also you are not define $id and $data in your function and change your update to

$this->db->update('reseller');

So finally your function would be

MODLE

 public function deduct($id)
    {
        $this->db->set('balance', 'balance-5',FALSE);
        $this->db->where('id' , $id);
        $this->db->update('reseller');
    }

CONTROLER

if($this->input->post('balance') === "Yes")
         {

            $this->reseller_m->deduct($id);/// pass youe id here

            //echo "goimng";
         }