unset()
unset() destroys the specified variables. The behavior of unset() inside of a function can vary depending on what type of variable you are attempting to destroy. If a globalized variable is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.
<?php
$machine = new Machine;
echo "The model number is : " . $machine->getModelNumber();
unset($machine);
echo "The model number is " . $machine->getModelNumber();
?>