运行AJAX脚本而无事件

I have

    <button type="button" onclick=" showUser($_SESSION["id"] )">Change Content</button>

Is there a way to do something like

if(isset($_SESSION["id"] )) showUser($_SESSION["id"] ); ?

showUser is an AJAX script that prints something.

I want it to run without any button press.

try

<?php if(isset($_SESSION["id"] )) {?>
<script> showUser('<?php echo $_SESSION["id"];?>');</script>
<?php }?>

if (isset($_SESSION['id']))
{
    echo '<script>showUser('.$_SESSION['id'].');</script>';
}

You are messing HTML, JS and PHP to one code. But yes, it's possible:

<script>
  <?php
    if (isset($_SESSION['id']) {
  ?>
    showUser(<?php echo $_SESSION['id']; ?>); // do js function call to ajax function
  <?php } ?>
/* rest of js */
</script>