When we need to swap the values of two variables, we use a third variable temporarily to hold the data between swapping.  Something like this:

<?php
$tmp = $a;
$a = $b;
$b = $tmp;
?>

Here is another way without using a temp or a third variable.

<?php
list($b, $a) = array($a, $b);
?>

And if you want to make it a function:

<?php
function swapValues(&$a, &$b) {
     list($b, $a) = array($a, $b);
 }
$a = 10;
$b = 20;
swapValues($a, $b);
?>