php chaining method error and confusion

Question

I'm learning PHP OOP, but now I'm stuck with an error and confused regarding the chained method. Here is my code

<?php
    class Car {
        public $tank;

        public  function fill($float) {
            $this-> tank += $float;
            return $this;
        }


        public  function ride($float) {
            $miles = $float;
            $gallons = $miles/50;
            $this-> tank -= ($gallons);
            return $this;
        }
    }


    $bmw = new Car(); 
    $tank = $bmw -> fill(10) -> ride(40);// -> tank;
    echo "The number of gallons left in the tank: " . $tank . " gal.";
?>

Now the problem is when used to call the functions without calling the Public variable tank then it's showing the following error message.

Catchable fatal error: Object of class Car could not be converted to string in C:\xampp\htdocs\oop\chain.php on line 33

In this case why should I call the public variable tank while I am calling the two functions? If I'm not assigning any value directly to the Public Variable tank then why should I call that variable..??

I am very confused with this

Answer 1

Your method ride returns an instance of the class Car so if you echo it you try to echo the existing class instance directly. You have two options now:

__toString() magic function

Inside of the class

function __toString() {
    return $this->tank;
}

The echo call

echo "The number of gallons left in the tank: " . $tank . "gal.";

http://php.net/manual/en/language.oop5.magic.php#object.tostring

Implement a getter function

Inside of the class

function getRemainingGallons() {
    return $this->tank;
}

The echo call

echo "The number of gallons left in the tank: " . $tank->getRemainingGallons() . " gal.";

Or edit the chained function

$tank = $bmw -> fill(10) -> ride(40) -> getRemainingGallons();

Make sure to choose a clear name for your method so that you always know what it does.

Answer 2

Replace the line

$tank = $bmw -> fill(10) -> ride(40);// -> tank;

with

$bmw->fill(10);    
$bmw->ride(40);    
$tank = $bmw->tank; 

And it will work as expected.

Answer 3

You can do this two ways,

1) Directly access public variable

$tank->tank

2) Create a getter method for this like

public function getVolume() {
    return $this->tank;
}

Then access this by following way

echo "The number of gallons left in the tank: " . $tank->getVolume() . " gal.";<br>
echo "The number of gallons left in the tank: " . $tank->tank . " gal.";