[Snipest] How can I get the id of a page and make it working in php function

Dear All,

I need to do a small thig but it does does not work
This is my code

echo $fileName = "images/produits/page_30_0_large.jpg";

if(file_exists($fileName)){
echo '<img src="[[++site_url]]'.$fileName.'" title="[[*pagetitle]]" />';
}else{
echo '<img src="[[++site_url]]images/produits/no_image_large.jpg" title="[[*pagetitle]]" />';
}

The function file_exists returns true, but when I replace the number 30 by [[*id]]
the function file_exists return false

Why [[*id]] make trouble?

Many thank

Ok, I found.
In my snipest, I created a variable $page_id

and in my template I call my snapist
[[mySnipest? $page_id=[[*id]]]]