http://modxcms.com/extras/package/745

getUrlParam.php

/** <?php
 *
 * getUrlParam
 *
 * A simple snippet to return a value passed through a URL parameter.
 *
 * @ author Paul Merchant
 * @ copyright 2010 Paul Merchant
 * @ version 1.0.0 - October 15, 2010
 * @ MIT License
 *
 * OPTIONS
 * name - The parameter name, defaults to p
 * int - (Opt) Set as true to only allow integer values
 * max - (Opt) The maximum length of the returned value, defaults to 20
 * default - (Opt) The value returned if no URL parameter is found
 *
 * Example: [[getUrlParam? &name=`pageid` &int=`1`]]
 *
**/

// set defaults
$name = $modx->getOption('name',$scriptProperties,'p');
$int = $modx->getOption('int',$scriptProperties,false);
$max = $modx->getOption('max',$scriptProperties,20);
$output = $modx->getOption('default',$scriptProperties,'');

// get the sanitized value if there is one
if (isset($_GET[$name])) {
    if ($int) {
        $value = intval($_GET[$name]);
    } else {
        if (strlen($_GET[$name]) > $max) {
            $value = filter_var(substr($_GET[$name],0,$max), FILTER_SANITIZE_STRING);
        } else {
            $value = filter_var($_GET[$name], FILTER_SANITIZE_STRING);
        }
    }
    $output = $value;
}

return $output;