opening a popup via a form in php

Hello everybody !!!
I’m not sure I’m porting in the right place, but, as part of it is javascript, I thought it would be OK here…
My problem :
I have a form, which posts informations using a php file. What I want is to have the page opened to a popup window…
Here is what I have so far :

<script language=”JavaScript” type=”text/javascript”>

function popupform(myform, windowname)
{
if (! window.focus)return true;
window.open(“messages.php”,”TAG”,”width=550,height=550,scrollbars=no”);
myform.target=windowname;
return true;
}
</script>

then, the form :
<form name=”form1″ method=”post” onSubmit=”popupform(this, ‘TAG’)”>

<p align=”center”>Veuillez choisir un pseudonyme :

<input type=”text” name=”name” maxlength=”30″ class=”pseudo” style=”BACKGROUND-COLOR:black;”>
<input name=”submit” type=”submit” class=”boutons” value=”Entrer dans le chat”>

</form>

what I get, when I post my things, is a stupid : “The requested method POST is not allowed for the URL /tag/chat/tag.htm “

I really don’t know what to do. I don’t understand why it won’t let me open a popup window…
Any solution would be really great !!!!!
Thanks a lot
Ness

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