Modifying PHP Session Variables with JavaScript

I have some implicit PHP functions which are to be called on particular front-end events; such as button onclicks. By doing so this should modify my PHP SESSION variable accordingly. However, both functions, setAlan() and setMark(), seem to be getting called even if I just want the first one to execute.

<?php
    session_start();

    function setAlan() {
        $_SESSION['passed_user']='alan';
    }
    function setMark() {
        $_SESSION['passed_user']='mark';
    }
?>

<script>
    function getAlan() { $(document.body).append('<?php setAlan(); ?>'); }
    function getMark() { $(document.body).append('<?php setMark(); ?>'); }
    function show() { alert(' <?php echo "{$_SESSION['passed_user']}" ?> '); }
</script>

<div class="pod">
    <div class="pod_title"><h2>Select a User</h2></div>
    <div>
        <input type="button" onclick="getAlan();" value="alan">
        <input type="button" onclick="getMark();" value="mark">
        <input type="button" onclick="show();" value="show">
    </div>
</div>

I don't understand why setMark() is being called automatically? By what I've written, I believe I am only defining the PHP functions, not calling them; I use JS for that. Am I missing something? Thanks in advance!