iFrame / Javascript不会更新

Question

I'm actually learning Javascript and especially Iframes.

So I made a form which is checked by request.php and then sent to my iframe via target : framename.php with a function wich update my iframe.

The problem is that my frame doesn't update, and I have no console message.

I show you the code :

index.php

<form id = "welcome" action = "request.php" target = "hello" method="post">
    <label for = "welcomeMsg"> Message : </label>
    <input id = "welcomeMsg" name = "welcomeMsg" type = "text"/>
    <input type = "submit" value = "send" />
</form>



<iframe src = "hello.php" id = "hello" name = "hello"></iframe>

<script>

function sayMsg(welcomeMsg){

    var frame  = document.getElementById('hello').contentDocument;
    frame.innerHTML = welcomeMsg;
}

</script>

request.php

<script>
window.top.window.sayMsg("<?php echo $_POST['welcomeMsg']; ?>");
</script>

hello.php (iframe)

<p id = "mess"> Hello </p>

You may think 'Why are you using a html page as an iframe because an id would work', it is because I will customise this hello.php.