sunrise sunset calculations for the north pole are surprising

sunrise.php

<?php
date_default_timezone_set('GMT');
$tim = strtotime("December 21");
$fmt = SUNFUNCS_RET_STRING;
$lat = 90;
$lng = 0;

for($i = 0; $i<182; $i++) {
    echo date(DATE_RFC822, $tim), "\t",
        date_sunrise($tim, $fmt, $lat, $lng), " ",
        date_sunset($tim, $fmt, $lat, $lng), "\n";
    $tim = strtotime("+1 day", $tim);
}