function ziyaretkaydet($firmaid){
$ip=$_SERVER['REMOTE_ADDR'];
$sonuc=mysql_query("select * from ziyaret where firmaid='$firmaid' and ip='$ip'");
if (mysql_num_rows($sonuc)<=0){
mysql_query("insert into ziyaret values('','$firmaid','$ip')");
mysql_query("update firmalar set ziyaret=ziyaret+1 where id='$firmaid'");
}
}

en iyi iyileştirme bana göre ;

function ziyaretkaydet($firmaid){
$ip=$_SERVER['REMOTE_ADDR'];
$sonuc=mysql_query("select id from ziyaret where firmaid='$firmaid' and ip='$ip'");
if (!$sonuc){
mysql_query("insert into ziyaret values('','$firmaid','$ip')");
mysql_query("update firmalar set ziyaret=ziyaret+1 where id='$firmaid'");
}
}