<script>
    $(function(){
        $(".like").click(function(){

            var postid = $(this).attr("id");
              $.ajax({
                type:'POST',
                url:'addlike.php',
                data:'id='+postid,
                success:function(data){
                   if(data=="you already liked this post"){
                       alert(data);
                     }
                   else{
                      $('a#'+postid).html(data);
                      }
                }
            });

        });
    });

</script>