##
##This will work for PHP 5.3 -

$url1="yahoo.com";
$url2="yahoo.co.in";
$url3="hotmail.org";

$domain1 = strstr($url1, '.', true);
$domain2 = strstr($url2, '.', true);
$domain3 = strstr($url3, '.', true);

echo $domain1, $domain2, $domain3;


##
## For Older PHP (<5.2.8), this code will follow -
$url1="yahoo.com";
$url2="yahoo.co.in";
$url3="hotmail.org";

$domain1 = substr($url1, 0, strpos($url1, '.') );
$domain2 = substr($url2, 0, strpos($url2, '.') );
$domain3 = substr($url3, 0, strpos($url3, '.') );

echo $domain1, $domain2, $domain3;