How to pass variables as stdin into command line from PHP

Question

I am trying to write a PHP script that uses the pdftk app to merge an XFDF with a PDF form and output the merged PDF to the user. According to the pdftk documentation, I can pass the form data in via stdin and have the PDF output to the stdout stream. The normal, file-not-stream way to use pdftk from the command line is:

pdftk blankform.pdf fill_form formdata.xfdf output filledform.pdf

to use streams on the command line, you'd enter:

pdftk blankform.pdf fill_form - output -

I have a couple of problems:

1) I have gotten pdftk to return output via stdout using an xfdf file (instead of stdin) like so:

    exec("pdftk blankform.pdf fill_form formdata.xfdf output -", $pdf_output);
    file_put_contents("filledform.pdf",$pdf_output);

But the pdf that it creates is corrupt, according to Adobe Reader and a quick peek at the file with a text editor shows that, at the very least, it is not setting the line endings where they should be. I have an identical PDF created by pdftk where it output to a file, and the pdf looks fine in the text editor, so I know that it's not pdftk that's outputting bad data.

2) I can not for the life of me figure out how to set the stdin stream in PHP so that I can use that stream as my input for pdftk. From what I'm reading on the PHP documentation, stdin is read-only, so how does anything ever get into that stream?

Ideally, I would like to keep this really simple and avoid using proc_open(). I attempted to use that function and wasn't very sucessful, which is probably my fault, not the function's, but really my goals are simple enough I'd rather avoid using robust functions I don't need.

Ideally my code would look something like:

 $form_data_raw = $_POST;
 $form_data_xfdf = raw2xfdf($form_data_raw); //some function that turns HTML-form data to XFDF

 $blank_pdf_form = "blankform.pdf";

 header('Content-type: application/pdf');
 header('Content-Disposition: attachment; filename="output.pdf"');

 passthru("pdftk $blank_pdf_form fill_form $form_data_xfdf output -);

Just a heads up, it is possible to put the actual xml string in the command line, but I've had very unreliable results with this.

Edit

With much help, I now understand that my real question was "how can pipe a variable to a command line execution in PHP". Apparently proc_open is the best way to go, or at least the most straightforward. Since it took me forever to figure this out and since my research on Google suggests others may be struggling, I'll post the code that specifically worked for my problem:

$blank_pdf_form = "blankform.pdf";
$cmd = "pdftk $blank_pdf_form fill_form - output -";

$descriptorspec = array(
   0 => array("pipe", "r"),
   1 => array("pipe", "w")
);

$process = proc_open($cmd, $descriptorspec, $pipes);

if (is_resource($process)) {

    //row2xfdf is made-up function that turns HTML-form data to XFDF
    fwrite($pipes[0], raw2xfdf($_POST));
    fclose($pipes[0]);

    $pdf_content = stream_get_contents($pipes[1]);
    fclose($pipes[1]);

    $return_value = proc_close($process);

    header('Content-type: application/pdf');
    header('Content-Disposition: attachment; filename="output.pdf"');
    echo $pdf_content;
}

Answer 1

I'm not sure about what you're trying to achieve. You can read stdin with the URL php://stdin. But that's the stdin from the PHP command line, not the one from pdftk (through exec).

But I'll give a +1 for proc_open()

---

<?php

$cmd = sprintf('pdftk %s fill_form %s output -','blank_form.pdf', raw2xfdf($_POST));

$descriptorspec = array(
   0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
   1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
   2 => null,
);

$process = proc_open($cmd, $descriptorspec, $pipes);

if (is_resource($process)) {
    // $pipes now looks like this:
    // 0 => writeable handle connected to child stdin
    // 1 => readable handle connected to child stdout

    fwrite($pipes[0], stream_get_contents(STDIN)); // file_get_contents('php://stdin')
    fclose($pipes[0]);

    $pdf_content = stream_get_contents($pipes[1]);
    fclose($pipes[1]);

    // It is important that you close any pipes before calling
    // proc_close in order to avoid a deadlock
    $return_value = proc_close($process);


    header('Content-type: application/pdf');
    header('Content-Disposition: attachment; filename="output.pdf"');
    echo $pdf_content;
}
?>