Ok, so you said you wanted to get a variable from HTML and send it to the PHP file to change data, that is what the variable 'varfromhtml' is.
You just need to give it a value you want to send, you can get the value of a div using jQuery eg
<div id="div">data</div>
var varfromhtml = $.("#div").val();
The variable 'varfromhtml' is now equal to 'h08sahdas', you can mess around with that to get it right for you.
Here is the final script:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script>
var varfromhtml = $.("#div").val();
$(document).ready(function(){
$.ajax({
method: "POST",
action: "myfile.php",
data: {
varfromhtml:varfromhtml
},
success: function(data){
$('#mydiv').val(data);
}
});
</script>