This simple function will remove any non-ASCII character. Feel free to fork and extend!

<?php
/**
 * Remove any non-ASCII characters and convert known non-ASCII characters
 * to their ASCII equivalents, if possible.
 *
 * @param string $string
 * @return string $string
 * @author Jay Williams <myd3.com>
 * @license MIT License
 * @link http://gist.github.com/119517
 */
function convert_ascii($string)
{
    // Replace Single Curly Quotes
    $search[]  = chr(226) . chr(128) . chr(152);
    $replace[] = "'";
    $search[]  = chr(226) . chr(128) . chr(153);
    $replace[] = "'";

    // Replace Smart Double Curly Quotes
    $search[]  = chr(226) . chr(128) . chr(156);
    $replace[] = '"';
    $search[]  = chr(226) . chr(128) . chr(157);
    $replace[] = '"';

    // Replace En Dash
    $search[]  = chr(226) . chr(128) . chr(147);
    $replace[] = '--';

    // Replace Em Dash
    $search[]  = chr(226) . chr(128) . chr(148);
    $replace[] = '---';

    // Replace Bullet
    $search[]  = chr(226) . chr(128) . chr(162);
    $replace[] = '*';

    // Replace Middle Dot
    $search[]  = chr(194) . chr(183);
    $replace[] = '*';

    // Replace Ellipsis with three consecutive dots
    $search[]  = chr(226) . chr(128) . chr(166);
    $replace[] = '...';

    // Apply Replacements
    $string    = str_replace($search, $replace, $string);

    // Remove any non-ASCII Characters
    $string    = preg_replace("/[^\x01-\x7F]/", "", $string);

    return $string;
}
?>