mod_layout and dynamic images

Hello,

I'd like to continue discussion about problem described
here 
http://lists.tangent.org/pipermail/mod_layout/2007-February/002237.html 
and here 
http://lists.tangent.org/pipermail/mod_layout/2004-March/002069.html

The problem is about dynamically changed Content-type header of the 
output document.
For example:
<?php
    header("Content-Type: image/png");
    $string = $_GET['text'];
    $im     = imagecreate(300,300);
    $orange = imagecolorallocate($im, 220, 210, 60);
    $px     = (imagesx($im) - 7.5 * strlen($string)) / 2;
    imagestring($im, 3, $px, 9, $string, $orange);
    imagepng($im);
    imagedestroy($im);
?>


This script shouldnt be processed by mod_layout, but it is.
I have output_buffering=On in php.ini file, but it doesnt help.
I cant use LayoutIgnoreURI

I'm using latest apache 2.2 and mod_layout 5.0.
Is there a way to avoid processing such scripts?
Maybe there's some workaround about this problem?