Como te comentaron más arriba la directiva es output_buffering, tienes que ponerla como Off ya que por defecto esta en 4096 (4KB):
Cita: ; Output buffering allows you to send header lines (including cookies) even
; after you send body content, at the price of slowing PHP's output layer a
; bit. You can enable output buffering during runtime by calling the output
; buffering functions. You can also enable output buffering for all files by
; setting this directive to On. If you wish to limit the size of the buffer
; to a certain size - you can use a maximum number of bytes instead of 'On', as
; a value for this directive (e.g., output_buffering=4096).
output_buffering = Off
Si en tu hosting te permiten modificar tu php.ini solo agrega esa directiva como Off y no tendras que enviar nada de contenido antes.
Saludos.