W-Shadow.com

Sometimes it is necessary to verify that a given page really contains a specific link. This is usually done when checking for a reciprocal link in link exchange scripts and so on.

Several things need to be considered in this situation :

• Only actual links count. A plain-text URL should not be accepted.
• Links inside HTML comments (<!– … –>) are are no good.
• Nofollow’ed links are out as well.

Here’s a PHP function that satisfies these requirements :

function contains_link($page_url, $link_url) {
	/* Get the page at page_url */
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $page_url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
 
    curl_setopt($ch, CURLOPT_USERAGENT, 
      'Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)');
 
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 30);
    curl_setopt($ch, CURLOPT_TIMEOUT, 60);
    curl_setopt($ch, CURLOPT_FAILONERROR, true);
 
    $html = curl_exec($ch);
    curl_close($ch);
 
    if(!$html) return false; 
	
    /* Remove HTML comments and their contents */ 
    $html = preg_replace('/<!--.*-->/i', '', $html);
    
    /* Extract all links */
    $regexp='/(<a&#91;\s&#93;+&#91;^>]*href\s*=\s*[\"\']?)([^\'\" >]+)([\'\"]+[^<>]*>)/i';
    if (!preg_match_all($regexp, $html, $matches, PREG_SET_ORDER)) {
	    return false; /* No links on page */
    };
    
    /* Check each link */
    foreach($matches as $match){
	    /* Skip links that contain rel=nofollow */	
	    if(preg_match('/rel\s*=\s*[\'\"]?nofollow[\'\"]?/i', $match[0])) continue;
	    /* If URL = backlink_url, we've found the backlink */
	    if ($match[2]==$link_url) return true;
    }
    
    return false;
}

/* Usage example */

if (contains_link('http://w-shadow.com/','http://w-shadow.com/blog/')) {
	echo 'Reciprocal link found.';
} else {
	echo 'Reciprocal link not found.';
};

… yep, another obscure topic covered, all done 😉
Now I wonder whether I should do another post like the Top 7 Things People Want To Kill to balance this out.