Why am I getting the 'Some data has already been output, can't send PDF' error with FPDF?

FPDF Output Error: Avoiding Data Output Before PDF Generation

The FPDF library requires that no output is produced before attempting to generate the PDF. When encountering the error "Some data has already been output, can't send PDF," it is essential to ensure that no other data is being echoed or printed prior to calling the FPDF methods.

As an example, a functional code snippet without any output looks like:

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

However, output such as spaces, carriage returns, or echo statements before calling the FPDF methods will result in the error:

 <?php
echo "About to create the PDF";
$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Arial', 'B', 16);
$pdf->Cell(40, 10, 'Hello World!');
$pdf->Output();
?>

In the context of Drupal, ensuring no output occurs before using FPDF is crucial. The Drupal module's code should be examined to eliminate any unintentional output that could interfere with FPDF's operation.

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