title: Always says the input is alphanumeric except when i submits nothing
body: <p>Here is a code. Whenever I submits anything the output says it's alphanumeric help me to get out of here.</p>
  
  <pre><code>&lt;?php
  
  $string ="$_POST[in]";
  
  if(ctype_alnum($string)) 
  {
  echo "'{$string}' is alphanumeric!";
  } else {
  echo "'{$string}' is NOT alphanumeric!";
  }
  
  ?&gt;
  &lt;form action="alphanum.php" method="post"&gt;
  &lt;h1&gt;Make Input:&lt;input name="in"&gt;&lt;span&gt;&lt;input type="submit" value="Check"&gt;&lt;/span&gt;&lt;/h1&gt;
  &lt;/form&gt;
  </code></pre>