what does unset do in php code example

Example 1: destroy php variable

// Destroy a variable
<?php
unset ($var1, $var2.... )
?>

Example 2: destroy php variable

<?php

$first = 100;
$second = 200;

$third = $first + $second;
echo "Sum = ".$third;
// Destroy a variable with unset function
unset($third);

//after delete the variable call it again to test
echo "Sum = ".$third;
?>