Convert PHP to PDF

Question

I need to convert HTML generated PHP boletos to PDF.

codeigniter-bolete

I tried to use the Dompdf library, as well as FPDF and finally MPDF (the latter, from what I read on some forums, would be better, for reasons of CSS compatibility).

My work is being in the Codeigniter framework.

Someone, please, who has already been through this problem can help me to solve this problem since I am getting the errors below:

Message: Cannot Modify header information - headers already sent by (output Started at C: Program Files (x86) Vertrigoserv www material application Libraries boleto include layout_bb.php:494)

mPDF error: Some data has already been output to browser, can’t send PDF file

And I’m having trouble generating the PDF.

public function index()
{
    $dados = array(
        // Informações necessárias para todos os bancos
        'dias_de_prazo_para_pagamento' => 5,
        'taxa_boleto'                  => 1,
        'pedido'                       => array(
            'nome'           => 'Serviços de Desenvolvimento Web',
            'quantidade'     => '10',
            'valor_unitario' => '80',
            'numero'         => 10000000025,
            'aceite'         => 'N',
            'especie'        => 'R$',
            'especie_doc'    => 'DM',
        ),
        'sacado'                       => array(
            'nome'     => 'João da Silva',
            'endereco' => 'Av. Meninas Bonitas, 777',
            'cidade'   => 'Sapiranga',
            'uf'       => 'RS',
            'cep'      => '93816-630',
        ),
        // Informações necessárias que são específicas do Banco do Brasil
        'variacao_carteira'            => '019',
        'contrato'                     => 999999,
        'convenio'                     => 7777777,
    );

    // Gera o boleto
    //$this->boleto->bb($dados);
    
    $this->load->helper('mpdf');
    $html = ($this->boleto->bb($dados));
    pdf_create($html, 'boleto' . date('d/m/y'), TRUE);
}