Fatal error: Call to undefined function: scandir()

Does anyone know why I would be getting this message?

Fatal error: Call to undefined function: scandir()

Code looks like this, and is working on other servers. I am using PHP5.

// Fetch alternate theme stylesheets
$alt_stylesheet_dir = CHILDTHEME_THEME_DIRECTORY . 'styles/';
$alt_stylesheets = array();

if (is_dir($alt_stylesheet_dir)) {
  $styles = scandir($alt_stylesheet_dir);
  foreach ($styles as $style) {
    if (strpos($style, '.css')) {
      $alt_stylesheets[] = $style;
    }
  }
}