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

convert_ascii.php

<?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; 
}
?>

convert_latin1_ascii.php

/**
 * Convert known non-ASCII latin1 characters to their ASCII equivalents, if possible.
 *
 * This code have been modified by Han Lin Yap by combining functions from 
 * http://gist.github.com/119517 and http://phpxref.com/xref/wiclear/inc/classes/utf8_helper.class.php.source.txt
 *
 * @param string $string 
 * @return string $string
 * @version 1.0 (2011-12-15)
 */
function fix_latin1_ascii($string) { 
	$conv = array(
		// Replace Single Curly Quotes
		chr(194).chr(145) => "'",
		chr(194).chr(146) => "'",
		// Replace Smart Double Curly Quotes
		chr(194).chr(147) => '"',
		chr(194).chr(148) => '"',
		// Replace Bullet
		chr(194).chr(149) => '*',
		// Replace En Dash
		chr(194).chr(150) => '--',
		// Replace Em Dash
		chr(194).chr(151) => '---',
		// Replace Middle Dot
		chr(194).chr(183) => '*',
		// Replace Ellipsis with three consecutive dots
		chr(194).chr(133) => '...',
	);
	return str_replace(array_keys($conv), array_values($conv), $string);
}