$icerik = 'Lorem ipsum dolor sit <a href="konu.php?id=55">amet</a>, consectetur adipiscing elit. Curabitur in vestibulum metus. Nunc a justo nibh. Proin commodo mollis dignissim. Proin in sapien vel nisi luctus <a href="konu.php?id=5">pulvinar</a> pulvinar ultrices quis nisi. ';
$icerik = preg_rreplace('~<a href="([^"]+)"([^>]*)>~i', '<a href="$1&sesid=' . $sesid . '"$2>');