Wrap a span tag around all instances in a string that are surrounded by 4 asterisks.

gistfile1.txt

public static function span_strings($string, $class = null) {
  /* Takes a string, and surrounds anything surrounded by 4 asterisks with
    * a span tag with an optional class name.
    *
    * If a class is specified in the second argument, it will add
    * a class to the span tag. 
  */

  $match_phrase       = "/\*\*([^**]*)\*\*/";
  $replace_string     = '<span'; 
  $replace_string    .= $class ? ' class="' . $class . '">' : '>';
  $replace_string    .= '${1}</span>';
  $new_string         = preg_replace($match_phrase, $replace_string, $string);

  return $new_string;
}