Which hook i need to choose for do this?

[SnakeV]

Hello,

I'm trying to setup a code for display ads only for visitors that come from a search engine.

This is the code that i need to put for check if a visitor come from a search engine or not it this:

PHP Code:

$ref = $_SERVER['HTTP_REFERER'];
$SE = array('/search?', 'images.google.', 'web.info.com', 'search.', 'del.icio.us/search', 'soso.com', '/search/', '.yahoo.');
foreach ($SE as $source) {
if (strpos($ref,$source)!==false) {
setcookie("sevisitor", 1, time()+3600, "/", ".YOURDOMAIN.com");
$sevisitor=true;
}
}
function fromasearchengine(){
global $sevisitor;
if ($sevisitor==true || $_COOKIE["sevisitor"]==1) {
return true;
}
return false;
} 


I want it to be displayed in Showthread.

Which hook i need to choose?.

Thanks!!