title: Check if variable is_undefined in PHP
body: <p>In PHP, I want to check if a variable has not been set/defined, <b>where setting a variable NULL is considered set/defined</b>.</p>
  
  <p>I'm aware everything here:
  <a href="http://php.net/manual/en/types.comparisons.php">http://php.net/manual/en/types.comparisons.php</a>
  including isset(), empty(), and is_null(). None of these appear to be what I'm looking for. Consider the following example: </p>
  
  <pre><code>&lt;?php 
  $myNull = null;
  echo 'isset($myNull): "'.isset($myNull).'"&lt;br /&gt;';
  echo '$myNull value = "'.$myNull . '"&lt;br /&gt;';
  
  echo "&lt;br /&gt;";
  
  echo 'isset($myUndefined): "'.isset($myUndefined).'"&lt;br /&gt;';
  echo '$myUndefined value = "'.$myUndefined . '"&lt;br /&gt;';
  ?&gt;
  </code></pre>
  
  <p>This example outputs something like:<br />
  isset($myNull): ""<br />
  $myNull value = ""<br /></p>
  
  <p>isset($myUndefined): ""<br />
  Notice: <b>Undefined</b> variable: myUndefined in C:\wamp\www\plm\temp4.php on line 9<br />
  $myUndefined value = ""<br /></p>
  
  <p>I want to know if a variable is <b>Undefined</b> as it says above in the notice. I want a function, call it "is_undefined", where</p>
  
  <pre><code>$myNull = null;
  is_undefined($myNull); // is false
  is_undefined($myUndefined); // is true
  </code></pre>
  
  <p>Anyone? Thanks in advance.</p>