Form Not Submitting After Disabled Button

I've got a form inside a PHP script that posts to itself, does some
processing on the PHP side, and if it looks good, PHP sends a HEADER
command that redirects the user out. All was fine until I added a
button disabler onSubmit to keep users from submitting twice. Now the
form disables the button as requested but just refreshes the page... it
doesn't get to the HEADER command.

Any idea what the problem might be?

<script language="Javascript">
function disable(theForm) {
theForm.send_email.disabled = true
return (true)
}
</script>

<form action="<?=$_SERVER[PHP_SELF]?t=$_GET[t]?>" method=post name=form
onSubmit="return disable(this)">