致命错误:num行

我有这个错误：

致命错误：已达到最大函数嵌套级别"1000"，正在中止！在C:''examplep''htdocs''progettoPersonale''libs''DbConnector.php的第47行

我有两个类，但是当我试图知道行数时，我会错误

<?php
class UserModel
{
    private $dbconn = null ;
    public function __construct()
    {
        $this->dbconn = new DbConnector();
    }

    public function checkLogin($username, $password ){
     $new_password=password_hash($password, PASSWORD_DEFAULT);
    $res =$this->dbconn->select(
        array('*'),
        'iscritti',
        "nome_utente='".$this->dbconn->escape($username)."' && password='".$this->dbconn->escape($new_password)."'"
    );
        $resault =$this->dbconn->num_rows($res);
        if($resault==1){
            return true ;
        }else{
            return false ;
        }
    }
}
<?php
class DbConnector
{  private $_dbconn = null;
    public function __construct()
    {
        $this->_dbconn= new mysqli(DB_HOST,DB_USER,DB_PASSWORD,DB_DATABASE);
        if (mysqli_connect_errno())
        {
            echo "Failed to connect to MySQL: " . mysqli_connect_error();
        }
    }
    public function select( $fields, $table, $conditions = '1' ) {
        $query = "SELECT ";
        $query .= implode( ', ', $fields );
        $query .= " FROM ";
        $query .= $table;
        $query .= " WHERE ";
        $query .= $conditions;
        var_dump($query);
        return $this->_dbconn->query($query);
    }
    public function insert($table,$fields,$values){
        $query = "INSERT INTO" .$table."";
        $query.= "(".implode(',', $fields).")";
        $query.= "VALUES" ;
        $query.= "(".implode(',', $values).")";
        return $this->_dbconn->query($query);
    }
    public function fetch_assoc($res){
        return $res->fetch_assoc();
    }
    public function query($query){
        return $this->_dbconn->query($query);
    }
    public  function escape($string){
        return $this->_dbconn->real_escape_string($string);
    }
    public function last_id(){
        return $this->last_id();
    }
    public function num_rows(){
        return $this->num_rows();
    }
    public function close_con(){
        $this->close_con();
    }
}