The simplest way would be to do this:
<xmp><?php include("file.txt"); ?></xmp>
But, you could also use something like this, which is more effective:
<?php
$fp = fopen("file.txt","r");
$text = fread($fp,filesize("file.txt"));
fclose($fp);
$text = ereg_replace("<","<",$text);
$text = ereg_replace(">",">",$text);
echo($text);
?>