seo-url.php

<?php

/** 
  * Return URL-Friendly string slug
  * @param string $string 
  * @return string 
  */
function seoUrl($string) {
    //Unwanted:  {UPPERCASE} ; / ? : @ & = + $ , . ! ~ * ' ( )
    $string = strtolower($string);
    
    //Strip any unwanted characters
    $string = preg_replace("/[^a-z0-9_\s-]/", "", $string);
    
    //Clean multiple dashes or whitespaces
    $string = preg_replace("/[\s-]+/", " ", $string);
    
    //Convert whitespaces and underscore to dash
    $string = preg_replace("/[\s_]/", "-", $string);
    
    return $string;
}

/** Another way */
function slugify($text)
{
  // replace all non letters or digits with -
  $text = preg_replace('/\W+/', '-', $text);

  // trim and lowercase
  $text = strtolower(trim($text, '-'));

  return $text;
}