Oh. That will complicate things. 
Ok. I tried it. Still not working. It looks like I can send some letters and some I can't. Php code:
<?php
include "php_serial.class.php";
$serial = new phpSerial;
$serial->deviceSet("COM7");
$serial->confBaudRate(9600);
$serial->confParity("none");
$serial->confCharacterLength(6);
$serial->confStopBits(1);
$serial->confFlowControl("none");
$serial->deviceOpen();
$serial->sendMessage();
sleep(5);
$slovo = $_POST["slovo"];
$serial->sendMessage($slovo);
sleep(5);
$serial->deviceClose();
header("Location: led.html");
?>
But, thank you very much.