getimagesize与路径中的变量

This is my code

$_SESSION['skin'] = 'kids';
$data = getimagesize("./skin/images/teaser/".$_SESSION['skin'].".jpg");

I'm getting this error:

mod_fcgid: stderr: PHP Warning:  getimagesize(./skin/images/teaser/skin.jpg): failed to open stream: No such file or directory

But I don't understand why the path is: ./skin/images/teaser/skin.jpg and not ./skin/images/teaser/kids.jpg

When I'm using var_dump on $_SESSION['skin'] just before getimagesize I get the correct output:

string(4) "kids"