<?php if ($_SERVER['REQUEST_METHOD'] == 'POST') {
print_r($_POST);
} else { ?>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Test</title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script>
$(document).ajaxSend(function (e, xhr, s) {
s.data = s.data ? s.data+'&' : '';
s.data += 'my_value=1';
})
$.post('/test.php'); // пустой $_POST
$.post('/test.php', {value: 0}); // $_POST с value и my_value
</script>
</head>
<body>
</body>
</html>
<?php } ?>