String value swap suing two variable without use third variable in php

<?php
 $a="value stored in a";
 $b="value stored in b";
 $a= $a.$b;  

    $b =substr($a,0,strlen($a)-strlen($b)) ;
    $a = substr($a,strlen($b));
    echo strlen($b);

    echo ("a = ".$a)
    echo ("b = ".$b);
    exit;
?>