title: Why does mb_convert_encoding fail?
body: <p>Why does </p>
  
  <pre><code>&lt;?php 
  
  echo "HELLO WORLD 1"; // shows
  error_reporting(E_ALL);
  echo "HELLO WORLD 2"; // shows
  print_r(mb_list_encodings()); // does not show
  echo "HELLO WORLD 3"; // does not show
  $result = mb_convert_encoding("apple", 'UTF-8');
  echo "HELLO WORLD 4"; // does not show;
  echo $result; // does not show;
  
  // no error what so ever displayed.
  
  ?&gt;
  </code></pre>
  
  <p>fail? What can make this function fail?</p>
  
  <p>I have a PHP web page that runs code and halts at this line, and returns HTTP 500 error.</p>
  
  <p>But I don't know why it fails. Any suggestion for where to check?</p>
  
  <p><strong>Update:</strong>
  Error Log shows</p>
  
  <pre><code>PHP Fatal error:  Call to undefined function mb_convert_encoding()
  </code></pre>