Wrap your array variable inside curly braces {}
like this
value="{$this->_user['user_login']} {$this->_class['user_login']}"
Here is a nice article on this.
From the article
Unfortunately, PHP doesn’t provide any direct means for calling functions or outputting expression results in HEREDOC strings. The problem here is that, unless the thing in the curly braces starts with a dollar sign, it can’t be recognized by PHP as something to be replaced.