在页面加载时提交ajax

So I am submitting my AJAX just before the page is loaded and calling it on the same page like this after ajax.

<input type="text" class="hidden" id="storeID" value="<?php echo $_GET['store']; ?>">
$(document).ready(function()
{
var store = $("#storeID").val();
$.ajax(
{
  url: '../user-style.php',
  method: 'POST',
  data: {"store":store}
});
});
<link rel="stylesheet" href="../user-style.php" media="screen" />

user-style.php

if(isset($_POST['store']))
{
$stmtgetstyle = $mysqli->prepare("SELECT * FROM store_style_configuration WHERE store_id=?");
$stmtgetstyle->bind_param("i", $_GET['store']);
$stmtgetstyle->execute();
$getstyle = $stmtgetstyle->get_result();
$style = $getstyle->fetch_assoc();
$stmtgetstyle->close();
}

But user-style.php isn't getting any data neither any thing is from database is coming.

You are posting your data in the ajax call, but are binding to the variable $_GET['store']. Just change that to $_POST['store'] and it should work. This should have also sent out a notice error stating there was an undefined index 'store' in $_GET on line .... This is why turning on error_reporting(E_ALL) is good when in development.

Edit: actually, on second thought, it might not have thrown an error because when using a variable by reference creates a variable if it doesn't exist and doesn't throw an error. I assume this is the same for undefined array indexes.

You'll want to pull the content of the CSS and return it. Now you can dynamically put it in a jQuery Object:

$.ajax({
    url: '../user-style.php',
    method: 'POST',
    data: {"store":store},
    success: function(data) {
        var $cssStyles = $('<style/>');
        $cssStyles.attr('type', 'text/css');
        $cssStyles.html(data);
        $cssStyles.appendTo($('head'));
    }
});

Another method would be to pass it to the current script via the URL parameters with $_GET:

<link rel="stylesheet" href="../user-style.php?store=store" media="screen" />

Or with jQuery and $_GET:

$(document).ready(function() {
    var store = $("#storeID").val();
    var $cssStyles = $('<style/>');
    $cssStyles.attr({ 'type': 'text/css', 'href': '../user-style.php?store=' + store });
    $cssStyles.appendTo($('head'));
});