understanding class inheritance

<?php

class car{
    var $wheels = 4;
    var $doors = 4;
    function wheelsdoors(){
        return $this->wheels + $this->doors;
        
    }
    
    
}


class CompactCar extends car{
    
    var $doors = 2;
        
}

$car1 = new car();
$car2 = new CompactCar();


echo $car1->wheels . "<br>";
echo $car1->doors . "<br>";
echo $car1->wheelsdoors(). "<br>";
echo  "<br>";
echo  "<br>";
echo  "<br>";


echo $car2->wheels . "<br>";
echo $car2->doors . "<br>";
echo $car2->wheelsdoors(). "<br>";




?>