$kugoster = "SELECT * FROM yazilar LEFT JOIN kullanicilar ON yazilar.yazarid = kullanicilar.kullaniciid WHERE yazilar.goster='1' DESC LIMIT 5"; 
    $kgoster = mysql_query($kugoster,$baglan);
ya da

    $kugoster = "SELECT * FROM yazilar WHERE goster='1' DESC LIMIT 5"; 
    $kgoster = mysql_query($kugoster,$baglan); 

    while($kgetir = mysql_fetch_object($kgoster)) { 
    $yazarid = $kgetir->yazarid;  
    
    $query = mysql_query("SELECT * FROM kullanicilar WHERE kullaniciid =".$yazarid);
    $row = mysql_fetch_array($query);
    echo $row['kullaniciadi'];

    }