Well, I got it
<?php
require("php_serial.class.php");
function Arduino($str){
$serial = new phpSerial();
$serial->deviceSet("COM7");
$serial->confBaudRate(9600);
$serial->confCharacterLength(8);
$serial->deviceOpen();
for ($i=0; $i<strlen($str); $i++){
$serial->sendMessage($str[$i]);
}
$serial->deviceClose();
}
Arduino($_POST['slovo']);
header("Location: led.html");
?>