这是我的代码：

<?php
include ("../connections.php");
require("fpdf181/fpdf.php");
ob_end_clean();
ob_start();
if (isset($_POST["btn_1"])) {
class PDF extends FPDF{

function Header(){
  global $title;
  $w = $this->GetStringWidth($title)+6;
  $this->SetX((210-$w)/2);
  $this->Image('fpdf181/logo.png',10,6,20);
  $this->SetFont('Times','B',30);
  $this->SetLineWidth(1);
  $this->Cell($w,10,$title,0,0,'C');
  $this->Ln(20);
}
function table(){

}
function layout(){


}

function footer(){
  $this->SetY(-15);
  $this->SetFont('Arial','I',8);
  $this->Cell(0,10,'Page '.$this->PageNo(),0,0,'C');
}
}
$pdf = new PDF();
$title = "MONTHLY SALES REPORT";
$pdf->SetTitle($title);
$pdf->AddPage();
$pdf->SetFont('Arial','B',12);
$pdf->SetFillColor(180,180,255);
$pdf->SetDrawColor(50,50,100);
$pdf->Cell(10,10,"Generic Name",1,0,'',true);
$pdf->Cell(60,10,'Branded Name',1,0,'',true);
$pdf->Cell(60,10,'Unit',1,0,'',true);
$pdf->Cell(65,10,'QTY Available',1,0,'',true);
$pdf->Ln();
$view_query = mysqli_query($connections, "SELECT * FROM product");
while ($row = mysqli_fetch_assoc($view_query))
{

$db_genericname = $row["genericname"];
$db_brandedname = $row["brandedname"];
$db_unit = $row["unit"];
$db_quantityavailable = $row["quantity_available"];

$pdf->Cell(10,10,$db_genericname,1);
$pdf->Cell(60,10,$db_brandedname,1); 
$pdf->Cell(60,10,$db_unit,1); 
$pdf->Cell(65,10,$db_quantityavailable,1); 
$pdf->Ln();
}
$pdf->Output();
ob_end_flush();
}
?>

这是我得到的错误：

致命错误：未捕获异常：FPDF错误：某些数据已经输出，无法在C：\ xampp \中发送PDF文件（输出从C：\ xampp \ htdocs \ SAD \ manager \ productrecords.php：1开始） htdocs \ SAD \ manager \ fpdf181 \ fpdf.php：271堆栈跟踪：＃0 C：\ xampp \ htdocs \ SAD \ manager \ fpdf181 \ fpdf.php（1052）：FPDF->错误（'某些数据有... . '）＃1 C：\ xampp \ htdocs \ SAD \ manager \ fpdf181 \ fpdf.php（999）：FPDF - > _ checkoutput（）＃2 C：\ xampp \ htdocs \ SAD \ manager \ productrecords.php（258） ：FPDF->输出（）＃3 在第271行的C：\ xampp \ htdocs \ SAD \ manager \ fpdf181 \ fpdf.php中引发

我知道该怎么做了，我尝试了我最好的搜索解决方案并尝试了它们但没有好处 . 一点帮助将不胜感激:)

php pdf fpdf