Send php variable with jQuery AJAX - php

Question

I´m using an image uploading jQuery script and a short php script. They´re both included inside of the mainsite.php file.
Everything is working fine expect the transfer of the php variable $usr_id.
How can I send the php variable $usr_id via AJAX to my server sided script correctly?
php code
$user = JFactory::getUser();
$usr_id = $user->get('id');
This is my try:
jQuery(function ($) {
$(document).ready(function () {
$("#but_upload").click(function () {
var fd = new FormData();
var files = $('#file')[0].files;
var userid = <?php echo $usr_id ?>;
if (files.length > 0) {
fd.append('file', files[0]);
fd.append('userid', userid);
$.ajax({
url: 'imageupload/upload.php',
type: 'post',
data: fd,
contentType: false,
processData: false,
success: function (response) {
if (response != 0) {
$("#img").attr("src", response);
} else {
alert('file not uploaded');
}
},
});
} else {
alert("Please select a file.");
}
});
});
})
Edit: this is the form field code
<form method="post" action="" enctype="multipart/form-data" id="myform">
<div >
<input type="file" id="file" name="file" />
<input type="button" class="button" value="Upload" id="but_upload">
</div>
</form>