This same thing is happening to me in a staging version of a site I am converting to MySQLi on my local server. It was an older version of forgotpassword.php that I am updating.
It is dumping an array of all of the user's information on the screen. I've attached the helper_php.php file used on this site.
I commented out line 216, but now the header warning moved on to line 217 and 306. I'm not seeing any white spaces, but I'm sure you will find the problem quickly.