Strips out inline styles like '<span style="font-family:arial;font-size:13px">'. Arguments work similarly to strip_tags().

strip_inline_styles.php

<?php

function strip_inline_styles($string, $allowed_styles = array()) {
  $pattern = '/style="([^"\']+)"/';
  $parsed = preg_replace_callback($pattern, function($match = NULL) use($allowed_styles) {
    // Get style attribute values
    $styles = $match[1];
    // Parse each sytle out
    $pattern2 = "/(?:([\w-]+?):([\w-]+);?)/";
    $parsed_styles = preg_replace_callback($pattern2, function($match2 = NULL) use($allowed_styles, $styles) {
      list($style, $key, $val) = $match2;
      // Is the style in the allowed styles?
      if (in_array($key, $allowed_styles)) {
        return $style;
      } 
      else {
        return '';
      }
    }, $styles);
    return (!empty($parsed_styles)) ? 'styles="' . $parsed_styles . '"' : '';
  }, $string); 

  return $parsed;
}

?>