click1.style.left = enemigo1.style.left + "px";   
click1.style.top = enemigo1.style.top + "px";
 
 

 $delete2 = "SELECT id_msj FROM messajes  WHERE id_usersend='$idder' AND id_userrecibe='$idreci' ORDER BY id_msj ASC limit 0,1";
 

 $delete2 = "SELECT id_msj FROM messajes  WHERE id_usersend='$idder' AND id_userrecibe='$idreci' ORDER BY id_msj DESC limit 0,1";
 

undefined index: usuario in ......LA RUTA... on line 20 

<form  method= "POST" action ="mensajesend.php">
    <input title = "usuario" id="usend" type = "text" name="usuario" placeholder = "Usuario" required maxlength="25" disabled /><br>
        <TEXTAREA title = "mensaje" type = "text" name = "mensaje" placeholder = "Escribe mensaje.." required maxlength="40"></TEXTAREA><br>
        <input type="submit"  value="Enviar"/>
   </form>
 

if($_SESSION['id']){
function antiscript($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}
        $usreci = antiscript($_POST['usuario']);
        $msj = antiscript($_POST['mensaje']);
................................................................
.................................................
......................................
.........
 
 

<button type="button" onclick="llamada()">Set the background image of div</button>
<div>
     <div id=bgc" class="c1" style="width:600px;background-image: url('assets/b1.png'); background-repeat: no-repeat;">
     <img class="caobj" src="assets/d5.png" width="400" height=300"/>
     </div>
</div>
 

 function llamada(){
          document.getElementById("bgc").style.backgroundImage = "url('assets/h1.png')";
    }
 
 

if (ereg("[^A-Za-z0-9]+",$_POST['usss'])) {
      $error12 = base64_encode("Esos caracteres no sirven");
echo "<script>location.href='/index.php?ffx=$error12'</script>";
}
 
 
_________________________________________________________________
 
if (preg_match("[^A-Za-z0-9]+",$_POST['usss'])) {
      $error12 = base64_encode("Esos caracteres no sirven");
echo "<script>location.href='/index.php?fxx=$error12'</script>";
}