Comprobar si hay conexión a Internet utilizando jQuery.Ajax con callback. --- Check internet connection using jQuery.Ajax with callback.

functions.js

$(document).ready(function() {
  
  var conn = true;
  checkConnection();
  
  function checkConnection() {
    	$.ajax({
  			url: 'http://yoursite.com/process.php',
  			async: false,
  			data: {'tag' : 'connection'}
  		})
  		.fail(function() { conn = false; })
  		.done(function() { conn = true; })
  }
})

process.php

<?php
 	header('Access-Control-Allow-Origin: *');

	$callback = isset($_GET['callback']) ? preg_replace('/[^a-z0-9$_]/si', '', $_GET['callback']) : false;
	header('Content-Type: ' . ($callback ? 'application/javascript' : 'application/json') . ';charset=UTF-8');

	if (isset($_REQUEST['tag']) && $_REQUEST['tag'] != "") {
		$tag = $_REQUEST['tag'];
		$json = array();

		if ($tag == 'connection') {
			$json['success'] = 'success';
			$json = json_encode($json);

			echo $callback ? "$callback($json)" : $json;
		}
  	}
?>