(Easy enough) What is the right code, to get username in plugin

[basketmen]

i have a custom plugin, here is below the content

if ($_POST['do'] == 'addmember')
{
if (empty($_POST['email']))
{
$_POST['emailconfirm'] = $_POST['email'] = 'username@mydomain.com';
}
else
{
$_POST['emailconfirm'] = $_POST['email'];
}
}

i want code in bold above to get member username




i had try put these code there

$username
$bbuserinfo[username]
$userinfo[username]
$userinfo['username']
$vbulletin->userinfo[username]
$vbulletin->userinfo['username']



but no one works, please help guys what is the right code to get member username in plugin