There are two examples in PHP MANUAL of ip2long() usage.
The first one is based on echo output and the second - on printf() function. These are examples with output:

<?
$ip = gethostbyname("www.php.net");
$out = "The following URLs are equivalent:<br>\n";
$out .= "http://www.php.net/, http://".$ip."/, and http://".ip2long($ip)."/<br>\n";
echo $out;
?>

The output of above is:
////////////////////////////////////////////////////////
The following URLs are equivalent:
http://www.php.net/, http://208.210.50.161/, and http://-791530847/
////////////////////////////////////////////////////////

I was confused on $ip output (i supposed ip-numbers are unsigned integers and cannot contain negative values like -791530847, echoed above). So, i've tryed to run the second example, wich uses printf() function:

<?php
$ip = gethostbyname("www.php.net");
printf ("%u\n", ip2long ($ip));
echo $out;
?>

The output is:
///////////
3503436449
///////////

I've changed code different ways, 've restarted machine, Apache server, but the difference remains. So, i've decided it's a bug either of echo or of ip2long() functions.