Yeah, delving back into my old days with assembler, you can do it using xor:

$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;
[download]

This swaps 2 variables without using a temporary, simply do it twice to swap three, e.g:

$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;
$b = $b ^ $c;
$c = $b ^ $c;
$b = $b ^ $c;
[download]

Hope this helps ;-)