$x = 10;

$y = 20;

The usual way of exchanging their variables is to use a temporary variable like the one below

$x = 10;

$y = 20;

$z = $x; // $z will be equal to 10 now which will serve as a temporary variable

$x = y; // $x will be equal to 20 now

$y = $z; // $y will be 10 now

Their is another way to do this without using temporary variable is the use of list() function

$x = 10;

$y = 20; 

list($x, $y) = array($y, $x);

You can also do this in multiple variables

$sDog1 = "Boston Terrier";

$sDog2 = "American Bulldog";

$sDog3 = "German Shepperd";

$sDog4 = "Great Dane";

list($sDog1, $sDog2, $sDog3, $sDog4) = array($sDog3, $sDog1, $sDog4, $sDog2);