title: How to reset a variable to NULL in PHP?
body: <p>I can use <code>isset($var)</code> to check if the variable is not defined or null. (eg. checking if a session variable has been set before)</p>
  
  <p>But after setting a variable, how do I reset it to the default such that <code>isset($var)</code> returns <code>false</code>?</p>