I have this simple program that I have and it doesn't create a pdf. I reduced it to the following to make things simple. The log file is world writable and I "touched" a file prince.log which is always empty. The web server log doesn't show any errors either.
<?php
error_reporting(E_ALL);
$html = '
<html>
<head>
<title>123</title>
</head><body>
test</body></html>';
include('include/prince.php');
$prince = new Prince('/home/epanagio/pdf/prince-7.1-freebsd63/prince');
$prince->setHTML(1);
$prince->setLog('/evan/prince.log');
header('Content-Type: application/pdf');
header('Content-Disposition: attachment; filename="evan.pdf"');
$prince->convert_string_to_passthru($html);
?>