how to pass a single variable to html from php page

you can use ajax to get the data like this . use this function in you user.html

<div id="var" type="hidden"/>
$(document).ready(function(){
    $.ajax({
            url: 'user_check.php',   // link your page
            type: 'POST',
            data: {
                //'key1': data1,
                // 'key2': data2
                //if you want to post somethig 
            },
            success: function(data) {
               // got success data
               $('#var').html(data);   // set value in the div
            },
            error: function(data) {
               // if error occure
                alert('Some Error Occurred. Please Try Later.');
            }
        });
});

in your user_check.php echo what you want to show . as ajax will display the data which will print

<? php
echo $var="1021";
?>

Note you may need to include jquery library for this

if jquery is not present . include this cdn

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>

if you want to know more about how .ajax work follow this refrence or refrence2

update

Another way to do this is include user.html after variable declaration .then user_check.php will

<?php
$var="1021";
include('user.html');
?>

and user.html will

<div id="var" type="hidden"><?php echo $var; ?></div>

I assume you're looking for something like this:

<? php
$var="1021";
?> 

<input id="var" type="hidden" value="<?php echo $var; ?>"/>

You can then access the value in Javascript. For example, using jQuery:

$('#var').val();