Before taking a deeper look, it appears that there's been a mixup of PHP and JavaScript variable types ... fix this first and if it still doesn't work let us know.
var $email;
var $senderName;
var $senderPhone;
var $senderEmail;
var $senderSubject;
var $senderMessage;
var $returnEmail;
var $header;
var $type = "text/plain";
var $characterSet = "iso-8859-1";
... should be ...
$email;
$senderName;
$senderPhone;
$senderEmail;
$senderSubject;
$senderMessage;
$returnEmail;
$header;
$type = "text/plain";
$characterSet = "iso-8859-1";
... since you don't use the
var keyword to initialize variables in PHP
Hope this helps, let us know