No index.php tem 1 campo input que envia um valor por ajax, que deve ser lido pelo cdb.php.

index.php:

<script type="text/javascript">         $  (document).ready(function(){             $  ("#formulario").on("submit", function(e){                 e.preventDefault();                 var cdb = $  ("#cdb").val();                 $  .ajax({                     url: "cdb.php",                     method: "POST",                     dataType: "html",                     data: cdb                 }).done(function(data){                     $  ("#cdb").val("");                     $  ("#cdb").focus();                     listar();                 }).fail(function(data){                     alert("Erro");                 });             });         }); </script> 

…

<form id="formulario" method="POST" action="" accept-charset="utf-8">         <div>             <?php echo rand(1, 100) ?>             <input id="cdb" type="text" name="cdb" class="validate" required autofocus onfocus="this.value='';">             <button type="submit">GRAVAR</button>         </div> </form> 

cdb.php

$  cdb = filter_input(INPUT_POST,'cdb');