I'm familiar with creating wordpress plugins. I tried hard to make [fpdf](http://www.fpdf.org) plugin to create pdf file from php contents, but IT didn't work 

I now that :
`require_once("fpdf.php");` should comes at the top of my file, and I think that is my main problem 

below is how to create a simple pdf file from php script using fpdf:

```
require('fpdf.php');
$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Arial','B',16);
$pdf->Cell(40,10,'Hello World!');
$pdf->Output();

```

What I should do to make it work?