How to swap two numbers without using third variable in php?

This is a very general question of any interview in any programming language. But most of the developers not aware about the logic of this question. 

Below is the solution of it see -

$first = 10;
$second = 20;

echo "Before swapping the number is First=".$first." second=".$second.'<br>';

$first  = $first + $second;
$second = $first - $second;
$first  = $first - $second;

echo "After swapping the number is First=".$first." second=".$second;

Output :

Before swapping the number is First=10 second=20
After swapping the number is First=20 second=10

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