php function to remove special characters

 php function to remove special characters

function clean($string) {
   $string = str_replace(' ', '-', $string); // Replaces all spaces with hyphens.
   return preg_replace('/[^A-Za-z0-9\-]/', '', $string); // Removes special chars.
}

how can I prevent multiple hyphens from being next to each other? 

and have them replaced with just 1? Thanks in advance! 

function clean($string) {
   $string = str_replace(' ', '-', $string); // Replaces all spaces with hyphens.
   $string = preg_replace('/[^A-Za-z0-9\-]/', '', $string); // Removes special chars.

   return preg_replace('/-+/', '-', $string); // Replaces multiple hyphens with single one.
}