Swap number without third variable.

<?php
	$a=12;
	$b=15;
	$a=$a+$b;
	$b=$a-$b;
	$a=$a-$b;
	
	echo $a."<br />".$b;
?>

OUTPUT:
$a=15
$b=12

Swap number with third variable.

<?php
	$a=12;
	$b=15;
	$c=$a;
	$a=$b;
	$b=$c;

	echo $a."
".$b;
?>

OUTPUT:
$a=15
$b=12