DevOps, CI/CD and Automation

hi! im trying to generate a pdf report using fpdf, but for some reason, i am not able to... here's my code im calling once a "sumbit" button is hit:

<?php

include_once('fpdf.php'); // Report Generation Tool
include_once('NewRequests.class.php');
include_once('Employees.class.php');
include_once('ProductCategories.class.php');

$cls_newReq = &new __cNewRequests;
$cls_employee = &new __cEmployees;
$cls_prodCat = &new __cProductCategories;

$_request = $cls_newReq->__displayRSMaster($_GET['docref'], $_GET['rsno']);
$_showItems = $cls_newReq->__displayRSDetails(0);

$_pdf = new FPDF();
$_pdf->AddPage();
$_pdf->SetFont('Arial','B',16);
$_pdf->Cell(40,10,'Hello World!');
$_pdf->Output();

?>

i am getting the ff: error and i am stuck here, so pls. kindly help me with this...:

FPDF error: Some data has already been output, can't send PDF file

the classes are my database manipulator to get the data... i did not use it (variables) to echo it but just to use it if that will work.

kindly help my guys! thanks!