PHP uses a temporary variable for combined assign-operators (unlike JavaScript), therefore the left-hand-side (target) gets evaluated last.
Input:
$a += $b + $c;
Meaning:
$a = ($b + $c) + $a;
Not:
$a = $a + ($b + $c);
This can be important if the target gets modified inside the expression.
$a = 0;
$a += (++$a) + (++$a); // yields 5 (instead of 4)