PHP implementation of variable value, how to handle

PHP implements Variable Value change
Write a piece of code in PHP, the implementation does not use the 3rd variable, Exchange $ A, $b value, $a, $b the initial value of their own.

------Solution--------------------
This is a lot of Baidu ah ....

These two numbers must be the integer number ah ...
------Solution--------------------
A = A+b
b = A A
A = a-b
------Solution--------------------

PHP Code

$a = "ABCD"; $b = "1234"; echo "a= $a when initialized, b= $b
"; $a = $a. $b, $b = strlen ($b), $b = substr ($a, 0, (strlen ($a)-$b)), $a = substr ($a, strlen ($b)), echo "a= $a after exchange, b= $b
";
------Solution--------------------
PHP has provided the perfect way to

$a = "ABCD";
$b = "1234";

List ($b, $a) = Array ($a, $b);

