<?php
if (is_int($_POST['postnumber']))
  {
  mysqli_query($connected,"INSERT INTO likes (userid, postid)
  VALUES ('".mysqli_real_escape_string($connected,$_SESSION['userid'])."',".$_POST['postnumber'].")");
  echo "Liked";
  }
else
  {
  ?>
  <form>
    <input type="hidden" name="postnumber" value="3456">
    <input type="submit" value="Like">
  </form>
  <?php
  }
?>