PHP函数作为if语句中的参数（面向对象）

when I run this:

if ($result->num_rows() > 0) {                                    
    while($row = $result->fetch_assoc()) {                     
            echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";   
    }
} else {
    echo "0 results";
}
$conn->close();

I get the following error:

Call to undefined method mysqli_result::num_rows()

I presume the error is from the num_rows() method but can't quite figure out what is wrong. As far as I know, objects call methods by using $obj->foo() in OOP but when I remove the parenthesis of num_row:

if ($result->num_rows > 0) {                                    
    while($row = $result->fetch_assoc()) {                     
            echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";   
    }
} else {
    echo "0 results";
}
$conn->close();

this block of code runs as expected.

The reason the second block of code works is because num_rows is an attribute of the object. Using num_rows() as a method results in a undefined method error because there is no method by that name.

An example:

class Dog {
    public weight;
    public age;

    public function __construct($weight, $age)
    {
        $this->weight = $weight;
        $this->age = $age;
    }

    public function bark()
    {
       ...
    }

    public function gain_weight()
    {
       $this->weight++;
    }
}

$dog = new Dog(10, 0);
$dog->gain_weight();
echo $dog->weight;

gain_weight is a method, but weight is an attribute of the $dog object.

On a side note, if ($result->num_rows > 0) is the same as if ($result->num_rows) since if $result->num_rows equals 0, the statement will evaluate to false.