WordPress Link Control Plugin

Hi there guys, I'm working on a wordpress plugin. Which gives user the control to manage keywords in the post by linking. It's that easy just give keyword and it's link it will be linked using Php function Preg_replace. What now i want to do in it is that preg_replace should only be executed when $pregurl == $currenturl
such as if you've the link of the post in the plugin, where you added words to be linked. Replace the words with with the link ONLY ON THAT POST. I need a wordpress coder help in it. Okay well, here's what I'm using..

function re_place_content_filter($content) {
	global $user_ID;
        global $wpdb;
	$pairs = get_re_pairs();

	$search = array();
	$place  = array();
$url="http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];


	foreach ($pairs as $re_place) {
		$search[] = "/" . $re_place->re_search . "/";

		if ( $re_place->restriction == 'auth' && !$user_ID ) {
				$place[] = $re_place->restr_otherwise;

		} else if ( $re_place->restriction == 'page' && !is_page() ) {
				$place[] = $re_place->restr_otherwise;

		} else if ( $re_place->restriction == 'post' && is_page() ) {
				$place[] = $re_place->restr_otherwise;

		} else {
			$place[] = $re_place->re_place;
		}
	}


		
		$sql = "SELECT re_url FROM `wa_re_place` WHERE re_url like '$url' "; 
		$replaces = $wpdb->get_results($sql);
 		echo $replaces;
		

if ($replaces == $url) {
echo 'a'; // just having that for testing purpose 
	$content = preg_replace($search, $place, $content, 1); // replacing every word only 1 time
	return $content;
}

else {

return $content; }

}

PHP:

What might be the problem?