At PHP2 it seemed to be very popular to use "while (expr); ... endwhile;".
Also the usage of "while (expr): ... endwhile;" produced an error:
    parse error, expecting `END_TAG' or `';'' or `'{'' on line 8
(Note the different delimiters ';' and ':' after "while ()"

With PHP3 the syntax changed to use ":" as the delimiter 
or (more likely) the users coded C syntax: "while (expr) { ... }".
Put the parser misses to detect a falsely used ';' which
causes an endless loop: (at least with PHP 3.0.7 and 3.0.8)

<?
        $num=3; $i=0;
        while ($i<$num);
                Echo "$i, "; 
                $i++;
        endwhile;
?>