How to automatically update google index when content website has changed - php

Question

Yesterday, i post an 'ad' on an ebay site (marktplaats.nl) and the ad was immediately visible in google when i search for this ad on Google.
How do they do this?
Because it is a very large site, i don't think they repost the sitemap. I have made such function to update the sitemap but don't know this is a good method. What i have is this (part of my library):
// returns false when input invalid, returns a number (array index) when url is invalid or // request fails, returns true when complete:
function suSubmitSitemap( $sXmlUrl, $asSearchEnginePingUrl = 'http://www.google.com/ping?sitemap=%s', $bContinueOnError = false )
{
if( !suIsValidString( $sXmlUrl ) || ( !suIsValidString( $asSearchEnginePingUrl ) && !suIsValidArray( $asSearchEnginePingUrl )))
{ return false; }
$a = (is_array($asSearchEnginePingUrl))?$asSearchEnginePingUrl:explode(',', $asSearchEnginePingUrl );
$sXmlUrl = urlencode( $sXmlUrl );
$ret = false;
foreach( $a as $i=>$sUrl )
{
$sUri = str_replace( '%s', $sXmlUrl, $sUrl );
$bValid = (!is_bool( strpos( $sUrl, '%s' )) && suGetUrlContent( $sUri ));
if( !$bValid )
{
if( !$bContinueOnError )
{ return $i; }
if( !is_array( $ret ))
{ $ret = array(); }
$ret[$i] = $sUri;
}
}
return ret;
}
Is this a safe way to do this (Google will not ban you when frequently called), when this is not the way to do it, does anyone know how implement such index update functionality in PHP?

Answer 1

Google will constantly crawl frequently updated/popular sites and update their search results. This has the benefit of making Google more relevant.
Resubmitting your site map will not help you get crawled as fast as Ebay. Get more visitors and post content more frequently to get Google to visit your site more often.
Also, check out:
http://www.google.com/support/webmasters/bin/answer.py?answer=48620
http://www.searchenginejournal.com/10-ways-to-increase-your-site-crawl-rate/7159/