Como remover caracteres especiais no PHP

Função para remover caracteres especiais no PHP, acentos, etc

function seo_friendly_url($string){
    $string = str_replace(array('[\', \']'), '', $string);
    $string = preg_replace('/\[.*\]/U', '', $string);
    $string = preg_replace('/&(amp;)?#?[a-z0-9]+;/i', '-', $string);
    $string = htmlentities($string, ENT_COMPAT, 'utf-8');
    $string = preg_replace('/&([a-z])(acute|uml|circ|grave|ring|cedil|slash|tilde|caron|lig|quot|rsquo);/i', '\\1', $string );
    $string = preg_replace(array('/[^a-z0-9]/i', '/[-]+/') , '-', $string);
    return strtolower(trim($string, '-'));
}

Função mais simples

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.
}