How to reset a variable to NULL in PHP?

As nacmartin said, unset will "undefine" a variable. You could also set the variable to null, however this is how the two approaches differ:

$x = 3; $y = 4;
isset($x);  // true;
isset($y);  // true;

$x = null;
unset($y);
isset($x);  // false
isset($y);  // false

echo $x;  // null
echo $y;  // PHP Notice (y not defined)

Use unset($var);