I'm trying to swap two number using operator but not able to swap the number please help to solve

Here, I’m trying to swap the number using PHP but not able to swap.
How to solve that problem.

My Code :

<?php  
$a = 45;  
$b = 78;  
echo "Before swapping:<br><br>";  
echo "a =".$a."  b=".$b;  
echo "<br/><br/>";
// Swapping Logic  
$a=$a+$b;  
$b=$a-$b;  
$a=$a+$b;  
echo "After swapping:<br><br>";  
echo "a =".$a."  b=".$b;  
?>  

I’m a beginner in php just learning about logic but getting exception so any body can help how to solve it?

>Solution :

Please replace your code

// Swapping Logic  

   $a=$a+$b;  
   $b=$a-$b;  
   $a=$a+$b; 

with

    $a=$a+$b;  
    $b=$a-$b;  
    $a=$a-$b;