PHP OOP: value property is guaranteed different?

Imagine this code:

class Test
{
    public $result;

    public function makeSum($a,$b)
    {
        $sum = $a+$b;
        $this->result = $sum;
    }

}

In a webserver.

At same exact moment, two browsers call a page that performs that operation. First one has $a=1 and $b=2 and second one $a=5 and $b=4.

Are we sure that for first page $result will be 3 and for second one 9? Or is there possibility that result could be "wrong" for the page asking?

I'm talking in particular for PHP, but question can be general...

I think that will be never problems, but I would know if it's true and why.

Thank you!