Given two variables, A and B say, exchange their values so that A has the previous value of B and B has the previous value of A.
The simple solution is to use a third variable:
<?
// set values for the variables
$a = 3;
$b = 7;
// save a to a new variable called c
$c = $a;
// a is now stored somewhere else, so override it with b
$a = $b;
// b is now stored in a, so update that to c
$b = $c;
// output the variables
echo 'A = ' . $a . ', B = ' . $b;
?>
Which produces:
A = 7, B = 3