<?php
 $isXmlHttpRequest = array_key_exists('X_REQUESTED_WITH', $_SERVER) &&
 $_SERVER['X_REQUESTED_WITH'] == 'XMLHttpRequest';
 
 if ($isXmlHttpRequest) {
 // is an Ajax request
 }
 else {
 // is not an Ajax request
 }
?>

and if you want to return json data to the ajax request thencode will be

Outputting JSON data for Ajax requests only 

 <?php
 $isXmlHttpRequest = array_key_exists('X_REQUESTED_WITH', $_SERVER) &&
 $_SERVER['X_REQUESTED_WITH'] == 'XMLHttpRequest';
 
 if ($isXmlHttpRequest) {
 $data = array(
 'foo' => 'bar'
 );
 
 header('Content-type: application/json');
 echo json_encode($data);
 exit;
 }
?>