$CurlConnect = curl_init();
curl_setopt($CurlConnect, CURLOPT_URL, 'https://geraboleto.sicoobnet.com.br/geradorBoleto/GerarBoleto.do');
curl_setopt($CurlConnect, CURLOPT_POST,   1);
curl_setopt($CurlConnect, CURLOPT_RETURNTRANSFER, 1 );
curl_setopt($CurlConnect, CURLOPT_POSTFIELDS, http_build_query(
    array(
        'dados_pra_passar_via_post' => '...',
        ...
    )
));
$result = curl_exec($CurlConnect);

curl_close ($CurlConnect);

$resultnew = str_replace('<img src="', '<img src="http://urlasboluta.com.br/', $result)

require("phpToPDF.php");

phptopdf_html($resultnew, '', 'legacy_html_example.pdf');

?>