How to read binary from pipe in PHP - php

Question

I'm trying to create a PNG from an SVG using Inkscape. I'm using Linux. the command,
cat in.svg | inkscape -z /dev/stdin -w 800 -h 475 -e /dev/stderr 2> out.png
works fine, but I would rather not write the output files on the server.
My code is,
<?php
$svg_data = file_get_contents('in.svg');
$descriptorspec = array(
0 => array("pipe", "r"), // stdin
1 => array("pipe", "w"), // stdout
2 => array("pipe", "w") // stderr
);
$pipes = array();
$process = proc_open("inkscape -z /dev/stdin -w 800 -h 475 -e /dev/stderr", $descriptorspec, $pipes);
if (is_resource($process)) {
fwrite($pipes[0], $svg_data);
fclose($pipes[0]);
$fil_data = stream_get_contents($pipes[2]);
fclose($pipes[1]);
fclose($pipes[2]);
proc_close($process);
file_put_contents("out.png", $fil_data);
}
?>
If I change the line,
$process = proc_open("inkscape -z /dev/stdin -w 800 -h 475 -e /dev/stderr", $descriptorspec, $pipes);
to
$process = proc_open("inkscape -z /dev/stdin -w 800 -h 475 -e out.png", $descriptorspec, $pipes);
The correct "out.png" is printed.
The current code writes a file, but the file is corrupted. It seems that it is corrupted from beginning to end, but it is about the right size.
I want the data (out.png) in $fil_data, but I don't want to read it from the disk drive.
Why is out.png corrupted, and how can I make the correct conversion without resorting to writing the disk.

Answer 1

Inkscape was putting some messages to stderr before writing the file. The same thing happened on stdout. The problem was fixed by skipping past the messages. So the line,
$fil_data = stream_get_contents($pipes[2]);
was changed to,
$fil_data = stream_get_contents($pipes[2], -1, 150);
If there is a better way, I would like to see it.