<?
$url = "www.yahoo.com";
$link = fopen("http://$url", "r");
echo "Link ($url): $link<BR>";

$url2 = "www.yahoo.com";
$link2 = fopen("http://$url2", "r");
echo "Link2 ($url2): $link2<BR>";
?>

Yields:
Link (www.yahoo.com):  ($link is empty)
Link2 (www.voyage.com): ($link2 is empty)

version:
PHP Version 4.0.6RC3

Configure:

'../configure' '--prefix=/usr' 
'--with-apxs=/usr/bin/apxs' 
'--with-regex=system'
'--with-config-file-path=/etc/php4/apache' 
'--disable-rpath' '--with-layout=GNU'
'--enable-memory-limit' 
'--enable-calendar' 
'--enable-sysvsem' 
'--enable-sysvshm'
'--enable-track-vars' 
'--enable-trans-sid' 
'--disable-debug' 
'--disable-static'
'--enable-bcmath' 
'--with-bz2' 
'--enable-ctype' 
'--with-curl=shared,/usr' 
'--with-db2'
'--with-ndbm' 
'--with-dom=shared,/usr' 
'--enable-exif' 
'--enable-filepro' 
'--enable-ftp'
'--with-gettext' 
'--with-gd=shared,/usr' 
'--with-jpeg-dir=shared,/usr'
'--with-xpm-dir=shared,/usr/X11R6' 
'--with-png-dir=shared,/usr' 
'--with-imap=shared,/usr'
'--with-ldap=shared,/usr' 
'--with-mcal=shared,/usr' 
'--with-mm' 
'--with-mhash=shared,/usr'
'--with-mysql=shared,/usr' 
'--with-pcre-regex=/usr' 
'--without-pgsql'
'--with-sablot=shared,/usr' 
'--enable-shmop' 
'--with-snmp=shared' 
'--enable-sockets'
'--with-sybase-ct=shared,/usr' 
'--with-ttf=shared,/usr' 
'--with-t1lib=shared,/usr'
'--enable-wddx' 
'--with-xml=/usr' 
'--with-expat-dir=/usr' 
'--enable-yp' 
'--with-zlib'