Please explain this php math error to me

Please explain this php math error to me

I'm learning PHP and playing around and I cant figure out why this outputs
150 and not 250
$num1 = 10;
$num2 = 15;
($num1 + $num2) * $num1 = $num3;
echo $num3;
I understand that my syntax was wrong and that I should put
$num3 = ($num1 + $num2) * $num1;
But I dont know what PHP's logic is here in getting 150
Thanks