PHP remove "reference" from a variable.

See explanation inline

$a = 1;    // Initialize it
$b = &$a;  // Now $b and $a becomes same variable with just 2 different names  
unset($b); // $b name is gone, vanished from the context  But $a is still available  
$b = 2;    // Now $b is just like a new variable with a new value. Starting new life.

$a = 1;
$b = &$a;

unset($b);
// later
$b = null;