php - What is += used for?

I think this is a dumb question but I could not find it on php. Why is a + with the = in the following code:

function calculateRanking()
{
    $created = $this->getCreated();

    $diff = $this->getTimeDifference($created, date('F d, Y h:i:s A'));


    $time = $diff['days'] * 24;
    $time += $diff['hours'];
    $time += ($diff['minutes'] / 60);
    $time += (($diff['seconds'] / 60)/60);

    $base = $time + 2;        

    $this->ranking = ($this->points - 1) / pow($base, 1.5);


    $this->save();
}

Is this so $time has all those values or rather it is adding all the values to $time?

Thanks

Answer

It's adding all those values to time.

something += somethingelse

is a shortcut for

something = something + somethingelse

-Adam