It is not an error under PHP to print a false value, which is all that the 
print() statement here would be doing once readdir() started returning
false. Therefore, the loop continues. For instance, the following bit of
code should never print 'Didn't print false':

<?php
if ( print( false ) ) {
   echo "Printed false<br>\n";
} else {
   echo "Didn't print false<br>\n";
}
?>

The following should do what you want:

<?php
$DH = opendir(".");
while ( $dir_entry = readdir( $DH ) ) {
   print "$dir_entry<br>\n";
}
?>