PHP [OOP] : Memory allocation for Inheritance

Question

Please see the code bellow:

class A {
    public x = 5;
    public y = 6;
    public z = 7;
}

class B extends A {
    public m = 1;
    public n = 2;
}

$a = new A();
$b = new B()

From the above code let $a is allocating x amount of memory and $b is allocating y amount of memory;

Now my question is which one is correct from bellow?

x > y

x < y

Answer 1

It should be public $x, $y, $z.

And $b takes up more memory because it has an instance of A inside.

Answer 2

You can investigate this by using the memory_get_usage function.

Answer 3

These are my numbers:

Starting allocation 62480
Allocated memory for new A() 328
Allocated memory for new B() 496

Thus x < y

These two class definitions are equivalent

class B extends A {
    public $m = 1;
    public $n = 2;
}

class C {
    public $x = 5;
    public $y = 6;
    public $z = 7;
    public $m = 1;
    public $n = 2;
}

Meaning that were you to change the definition of B into that of C then the memory usage would be the exact same for using new B() or new C().

To run it yourself use this code (as an example)

$start = memory_get_usage();
echo "Starting allocation $start\n";
$a = new A();
$diff = memory_get_usage() - $start;
echo "Allocated memory for new A() $diff\n";
$b = new B();
$diff = memory_get_usage() - $start - $diff;
echo "Allocated memory for new B() $diff\n";