Description:
------------
PHP cannot open, read from, or write to arbitrary file descriptors, a feature that is necessary for communications with certain utilities that expect input or output on certain file descriptors.  When programming in C, this functionality is provided by fdopen().  A feature request was filed over seven years ago for PHP to have a similar capability through the php:// stream wrapper (#26158), but the bug was marked as bogus for no adequately defined reason.

The alternative to being able to use a fdopen()-like function is to open the file descriptor as a named file through /dev/fd/ or /proc/self/fd/.  This works in C using fopen("/dev/fd/<descriptor>", "w"), but it does not work in PHP because PHP incorrectly attempts to dereference the pseudo-symlink before opening it.

I have a C program and a PHP script that both attempt to do the same thing: open file descriptor 3 for writing using /dev/fd/3, read until EOF from standard in and write to the file descriptor.  Both should be run as:

(executable) 3>&1 | cat

Test script:
---------------
PHP script:
<?php
$file = fopen("/dev/fd/3", "w");

if (!$file) exit(1);

$input = stream_get_contents(STDIN);
fwrite($file, $input);
fclose($file);

C program:
#include <stdio.h>

int main() {
  char buf[8192];
  int count;
  FILE *file;

  file = fopen("/dev/fd/3", "w");

  if (file == NULL) {
    perror("Opening /dev/fd/3");
    return 1;
  }

  while ((count = fread(buf, 1, sizeof(buf), stdin)) > 0) {
    fwrite(buf, count, 1, file);
    if (feof(stdin)) break;
  }
  return 0;
}

Expected result:
----------------
Input should be mirrored to file descriptor 3; when called using the command line above, it should be mirrored to standard output.

Actual result:
--------------
I get the following warning:

PHP Warning:  fopen(/dev/fd/3): failed to open stream: No such file or directory in test.php on line 2

An strace of the PHP process shows the incorrect behavior:

lstat("/dev/fd/3", {st_mode=S_IFLNK|0300, st_size=64, ...}) = 0
readlink("/dev/fd/3", "pipe:[637257]", 4096) = 13
lstat("/dev/fd/pipe:[637257]", 0x7fff76ae8140) = -1 ENOENT (No such file or directory)
lstat("/dev/fd", {st_mode=S_IFLNK|0777, st_size=13, ...}) = 0
readlink("/dev/fd", "/proc/self/fd"..., 4096) = 13
lstat("/proc/self/fd", {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0
lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=64, ...}) = 0
readlink("/proc/self", "28234"..., 4096) = 5
lstat("/proc/28234", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
lstat("/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
open("/proc/28234/fd/pipe:[637257]", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 ENOENT (No such file or directory)