Simple php model

User.php

<?php

class Customer 
{
    private $id;
    private $fields;

    public function __construct()
    {
        $this->id = null;
        $this->fields = array(
            'code'              => '',
            'usrname'           => '',
            'usrpwd'            => '',
            'token'             => '',
            'name'              => '',
            'title'             => '',
            'first_name'        => '',
            'last_name'         => '',
            'email'             => '',
            'mobile'            => '',
            'address'           => '',
            'status'            => '',
            'created_at'        => '',
        );
    }

    public function __get($field)
    {
        if($field == 'id') {
            return $this->id;
        }else {
            return $this->fields[$field];
        }
    }

    public function __set($field, $value) 
    {
        if($field == 'id') {
            $this->id = $value;
        }else {
            $this->fields[$field] = $value;
        }
    }

    public static function getByusrpwd($usrpwd)
    {
        $c = new Customer();
        $conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
        $sql = "SELECT * FROM customer WHERE usrpwd = '$usrpwd' ";
        $result = $conn->query($sql);
        if(mysqli_num_rows($result)){
            $row = mysqli_fetch_assoc($result);
            foreach ($row as $key => $value) {
                $c->$key = $value;
            }
        }
        $conn->close();
        return $c;
    }

    public static function getByCode($code)
    {
        $c = new Customer();
        $conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
        $sql = "SELECT * FROM customer WHERE code = '$code' ";
        $result = $conn->query($sql);
        if(mysqli_num_rows($result)){
            $row = mysqli_fetch_assoc($result);
            foreach ($row as $key => $value) {
                $c->$key = $value;
            }
        }
        $conn->close();
        return $c;
    }

    public function save()
    {
        if(!is_null($this->id)) {   
            $conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

            $keys = array_keys($this->fields);
            $last_key = array_pop($keys);

            $sql = "UPDATE customer SET";
            foreach ($keys as $index => $key) {
                $sql .= " " . $key . "='" . $this->$key . "', ";
            }
            $sql .= $last_key . "='" . $this->$last_key . "' ";
            $sql .= " WHERE id = '$this->id'";

            $result = $conn->query($sql);
 	       $conn->close();
            return $result;
        }
        // TODO: when id is null, create a new user into DB.
    }
}

$sql = "UPDATE customer SET $key[0] = {$this->$key}";
for (int i = 1; i < count($key); i++) {
 $sql .= ", $key = { $this->$key}";
}
$sql .= " WHERE id = '$this->id'";