$(document).ready(function(){
$('#listeler').on('change',function(){
var new_val = $("#listeler option:selected").val();
$.ajax({
type: 'post',
url: 'post.php',
data: { new: new_val },
dataType: 'text',
success: function(cevap){
document.getElementById('ulke').setAttribute("placeholder", cevap);
document.getElementById('ucret').setAttribute("placeholder", cevap);
document.getElementById('stok').setAttribute("placeholder", cevap);
}
});
});
)};

<?php
include('config.php');
if($_POST){
$liste = $_POST['new'];
$sorgu=$conn->prepare('SELECT *FROM numaralar WHERE no_id=:no_id');
$sorgu->execute(array(':no_id' => $liste));
$sorgula=$sorgu->fetch(PDO::FETCH_ASSOC);
$fiyat = $sorgula['no_fiyat'];
$adet = $sorgula['no_adet'];
$ulke = $sorgula['no_ulke'];
echo $fiyat.' ';
echo $adet.' ';
echo $ulke.' ';
?>