How to use TCPDF in conjunction with static attributes?

I'm having problems using static attributes to display the texts with TCPDF ... Trying to execute the codes below, I'm getting the following error:

TCPDF ERROR: Some data has already been output, can not send PDF file

I really need help !!

No printPdf.php

class printPdf extends conn {
    private $text1;
    private $text2;

    public function setText1() {...}
    public function setText2() {...}
    public function callMtds() {
        $this->setText1();
        $this->setText2();
    }

    public function getText1() {
        return $this->text1;
    }
    public function getText2() {
        return $this->text2;
    }

}
$Nt = new printPdf();
$Nt->callMtds();

No printPdfStatic.php

require_once 'printPdf.php';
cass printPdfStatic extends printPdf {
    public static $text1;
    public static $text2;

    public function setAttr() {
        self::$text1 = $this->getText1();
        self::$text2 = $this->getText2();
    }
}
$Nt = new setAttr();

No MYPDF.php

require_once('tcpdf.php');
class MYPDF extends TCPDF {
  //Page header
  public function Header() {
    // get the current page break margin
    $bMargin = $this->getBreakMargin();
    // get current auto-page-break mode
    $auto_page_break = $this->AutoPageBreak;
    // disable auto-page-break
    $this->SetAutoPageBreak(false, 0);
    // set background image
    $img_file = '../img/boleto_bancario.jpg';
    $this->Image($img_file, 0, 0, 210, 297, '', '', '', false, 300, '', false, false, 0);
    // restore auto-page-break status
    $this->SetAutoPageBreak($auto_page_break, $bMargin);
    // set the starting point for the page content
    $this->setPageMark();
  }
}

No printPdf_Control.php

require_once 'printPdfStatic.php';//O problema aparece já quando faço a requisição do arquivo onde estão os atributos estáticos...
require_once 'MYPDF.php';

// create new PDF document
$pdf = new MYPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);



// set document information
$pdf->SetCreator(PDF_CREATOR);
$pdf->SetAuthor('Victor Almeida');
$pdf->SetTitle('Teste PDF 01');
$pdf->SetSubject('Iniciando com o uso de PDFs');
$pdf->SetKeywords('TCPDF, PDF, example, test, guide');

// set header and footer fonts
$pdf->setHeaderFont(Array(PDF_FONT_NAME_MAIN, '', PDF_FONT_SIZE_MAIN));

// set default monospaced font
$pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED);

// set margins
$pdf->SetMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT);
$pdf->SetHeaderMargin(0);
$pdf->SetFooterMargin(0);

// remove default footer
$pdf->setPrintFooter(false);

// set auto page breaks
//$pdf->SetAutoPageBreak(TRUE, PDF_MARGIN_BOTTOM);
$pdf->SetAutoPageBreak(FALSE, 0);

// set image scale factor
$pdf->setImageScale(PDF_IMAGE_SCALE_RATIO);

// set some language-dependent strings (optional)
if (@file_exists(dirname(__FILE__).'/lang/por.php')) {
    require_once(dirname(__FILE__).'/lang/por.php');
    $pdf->setLanguageArray($l);
}

// ---------------------------------------------------------

// set font
$pdf->SetFont('times', 'b', 8);

// remove default header
$pdf->setPrintHeader(false);

// add a page
$pdf->AddPage();


// -- set new background ---

// get the current page break margin
$bMargin = $pdf->getBreakMargin();
// get current auto-page-break mode
$auto_page_break = $pdf->getAutoPageBreak();
// disable auto-page-break
$pdf->SetAutoPageBreak(false, 0);
// set bacground image
$img_file = '../img/boleto_bancario.jpg';
$pdf->Image($img_file, 0, 0, 210, 297, '', '', '', false, 300, '', false, false, 0);
// restore auto-page-break status
$pdf->SetAutoPageBreak($auto_page_break, $bMargin);
// set the starting point for the page content
$pdf->setPageMark();


// Print a text

$pdf->Text(108.0, 96.6, printPdfStatic::$text1);

$pdf->Text(2.0, 103.0, printPdfStatic::$text2);



// ---------------------------------------------------------

//Close and output PDF document
$pdf->Output('Teste_PDF_01.pdf', 'I');