RomanLaboiko, лови гк
$lol=mysql_fetch_assoc(mysql_query('SELECT `id` FROM `links` ORDER BY `id` DESC LIMIT 1'));
if(isset($_POST['next'])){

$id=isset($_POST['id'])?$_POST['id']-1:null;
}else{
$id=$lol['id'];
}
$site=mysql_fetch_assoc(mysql_query("SELECT * FROM `links` WHERE `id`='".abs(intval($id))."' ORDER BY id DESC LIMIT 1";
if($site){
$id=$site['id'];

echo '<div class="post">';
echo '<form action="" method="post">';
echo '<input type="hidden" name="id" value="'.$site['id'].'">';
echo '<input class="btn" type="submit" name="next" style="color: lime;" value="Следущая ссілка"></form>';

}