title: attempt to use array_map in a class is resulting in a warning
body: <p>A new class has been created for a PHP project as follows:</p>
  
  <pre><code>class Cleanse
  {
  
      # trims leading and trailing spaces
      public static function trimmer($values)
      {       
          return is_array($values) ?
                      array_map('trimmer', $values) :
                      trim($values);
      }
  
  }
  </code></pre>
  
  <p>However, when trying to use this functionality like so:</p>
  
  <pre><code>$values = Cleanse::trimmer($_POST);
  </code></pre>
  
  <p>the following warning message is returned:
  <strong>Warning: array_map() expects parameter 1 to be a valid callback, function 'trimmer' not found or invalid function name in (class file path) on line 41.</strong></p>
  
  <p>What is wrong with this code and/or this approach?</p>