From various Internet posts and recent experience, I have observed that you cannot rely on proc_close returning the accurate return code of the child process. The return code also depends on wether or not you read from the stdout/stderr pipes, as my example shows. I work around this by writing the exit code to an additional file descriptor.

<?
$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 => array('pipe', 'w'), // stderr is a pipe that the child will write to
    );
    $proc = @proc_open("/bin/ls -l /etc/passwd", $descriptorspec, $pipes);
    fclose($pipes[0]);
    $output = array();
    while (!feof($pipes[1])) array_push($output, rtrim(fgets($pipes[1],1024),"\n"));
    fclose($pipes[1]);
    while (!feof($pipes[2])) array_push($output, rtrim(fgets($pipes[2],1024),"\n"));
    fclose($pipes[2]);
    $exit=proc_close($proc);
    print_r($output);
    echo "exitcode $exit\n\n";

$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 => array('pipe', 'w'), // stderr is a pipe that the child will write to
    );
    $proc = @proc_open("/bin/ls -l /etc/passwd", $descriptorspec, $pipes);
    fclose($pipes[0]);
    fclose($pipes[1]);
    fclose($pipes[2]);
    $exit=proc_close($proc);
    echo "exitcode $exit\n\n";

$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 => array('pipe', 'w'), // stderr is a pipe that the child will write to
       3 => array('pipe', 'w'), // stderr is a pipe that the child will write to
    );
    $proc = @proc_open("/bin/ls -l /etc/passwd;echo $? >&3", $descriptorspec, $pipes);
    fclose($pipes[0]);
    $output = array();
    //comment next line to get correct exicode
    while (!feof($pipes[1])) array_push($output, rtrim(fgets($pipes[1],1024),"\n"));
    fclose($pipes[1]);
    while (!feof($pipes[2])) array_push($output, rtrim(fgets($pipes[2],1024),"\n"));
    fclose($pipes[2]);
    if (!feof($pipes[3])) $output['exitcode']=rtrim(fgets($pipes[3],5),"\n");
    fclose($pipes[3]);
    proc_close($proc);
    print_r($output);
?>

Outputs on my system:

Array
(
    [0] => -rw-r--r--  1 root root 1460 2005-09-02 09:52 /etc/passwd
    [1] =>
    [2] =>
)
exitcode -1

exitcode 1

Array
(
    [0] => -rw-r--r--  1 root root 1460 2005-09-02 09:52 /etc/passwd
    [1] =>
    [2] =>
    [exitcode] => 0
)

Regarding: "Returns the termination status of the process that was run. In case of an error then -1 is returned."

This is, at best, misleading. It returns:
* -1 on error,
* WEXITSTATUS(status) if WIFEXITED(status) is true, or 
* status if WIFEXITED(status) is false,
where status is the status parameter of waitpid().

This makes it impossible to differentiate between a relatively normal exit or a termination by signal, and reduces the value of the proc_close return code to a binary one (ok / something broke).

This can be seen in proc_open_rsrc_dtor() in ext/standard/proc_open.c (PHP 5.4.44, 5.6.12).

It seems that if you configured --enable-sigchild when you compiled PHP (which from my reading is required for you to use Oracle stuff), then return codes from proc_close() cannot be trusted.

Using proc_open's Example 1998's code on versions I have of PHP4 (4.4.7) and PHP5 (5.2.4), the return code is always "-1".  This is also the only return code I can cause by running other shell commands whether they succeed or fail.

I don't see this caveat mentioned anywhere except on this old bug report -- http://bugs.php.net/bug.php?id=29123

Under PHP/4.3.3RC2, in case of two processes 
these function may hangs. Work around is not use 
proc_close, or put it after all fcloses done. 

For example, this code hangs.

$ph1 = proc_open("cat",
 array(0=>array("pipe","r"),1=>array("pipe","w")),
 $pipes1);
$ph2 = proc_open("cat",
 array(0=>array("pipe","r"),1=>array("pipe","w")),
 $pipes2);

fclose($pipes1[0]); fclose($pipes1[1]); proc_close($ph1);
fclose($pipes2[0]); fclose($pipes2[1]); proc_close($ph2);

This code worked for me:

$ph1 = proc_open("cat",
 array(0=>array("pipe","r"),1=>array("pipe","w")),
 $pipes1);
$ph2 = proc_open("cat",
 array(0=>array("pipe","r"),1=>array("pipe","w")),
 $pipes2);

fclose($pipes1[0]); fclose($pipes1[1]); 
fclose($pipes2[0]); fclose($pipes2[1]); 
proc_close($ph1); proc_close($ph2);

Consider the following pseudo code:

$SOME_PROCESS = proc_open(/* something here */);
...
$status = proc_get_status($SOME_PROCESS);
...
$exitCode = proc_close($SOME_PROCESS);

If the external program has exited on its own before the call to proc_get_status, then $exitCode == -1

So consider using:
$actualExitCode = ($status["running"] ? $exitCode : $status["exitcode"] );