Redirect pipe acquired by proc_open() to file for remainder of process duration

Say, in PHP, I have bunch of unit tests. Say they require some service to be running.

Ideally I want my bootstrap script to:

start up this service
wait for the service to attain a desired state
hand control to the unit-testing framework of choice to run the tests
clean up when the tests end, gracefully terminating the service as appropriate
set up some way of capturing all output from the service along the way for logging and debugging

I'm currently using proc_open() to initialize my service, capturing the output using the pipe mechanism, checking that the service is getting to the state I need by examining the output.

However at this point I'm stumped - how can I capture the rest of the output (including STDERR) for the rest of the duration of the script, while still allowing my unit tests to run?

I can think of a few potentially long-winded solutions, but before investing the time in investigating them, I would like to know if anyone else has come up against this problem and what solutions they found, if any, without influencing the response.

Edit:

Here is a cutdown version of the class I am initializing in my bootstrap script (with new ServiceRunner), for reference:

<?php


namespace Tests;


class ServiceRunner
{
    /**
     * @var resource[]
     */
    private $servicePipes;

    /**
     * @var resource
     */
    private $serviceProc;

    /**
     * @var resource
     */
    private $temp;

    public function __construct()
    {
        // Open my log output buffer
        $this->temp = fopen('php://temp', 'r+');

        fputs(STDERR,"Launching Service.\n");
        $this->serviceProc      = proc_open('/path/to/service', [
            0 => array("pipe", "r"),
            1 => array("pipe", "w"),
            2 => array("pipe", "w"),
        ], $this->servicePipes);

        // Set the streams to non-blocking, so stream_select() works
        stream_set_blocking($this->servicePipes[1], false);
        stream_set_blocking($this->servicePipes[2], false);

        // Set up array of pipes to select on
        $readables = [$this->servicePipes[1], $this->servicePipes[2]);

        while(false !== ($streams = stream_select($read = $readables, $w = [], $e = [], 1))) {
            // Iterate over pipes that can be read from
            foreach($read as $stream) {
                // Fetch a line of input, and append to my output buffer
                if($line = stream_get_line($stream, 8192, "\n")) {
                    fputs($this->temp, $line."\n");
                }

                // Break out of both loops if the service has attained the desired state
                if(strstr($line, 'The Service is Listening' ) !== false) {
                    break 2;
                }

                // If the service has closed one of its output pipes, remove them from those we're selecting on
                if($line === false && feof($stream)) {
                    $readables = array_diff($readables, [$stream]);
                }
            }
        }

        /* SOLUTION REQUIRED SOLUTION REQUIRED SOLUTION REQUIRED SOLUTION REQUIRED */
        /* Set up the pipes to be redirected to $this->temp here */

        register_shutdown_function([$this, 'shutDown']);
    }

    public function shutDown()
    {
        fputs(STDERR,"Closing...\n");
        fclose($this->servicePipes[0]);
        proc_terminate($this->serviceProc, SIGINT);
        fclose($this->servicePipes[1]);
        fclose($this->servicePipes[2]);
        proc_close($this->serviceProc);
        fputs(STDERR,"Closed service\n");

        $logFile = fopen('log.txt', 'w');

        rewind($this->temp);
        stream_copy_to_stream($this->temp, $logFile);

        fclose($this->temp);
        fclose($logFile);
    }
}

Solution

What I ended up doing, having reached the point where the service had been initialized correctly, was to redirect the pipes from the already opened process as the standard input to a cat process per-pipe, also opened by proc_open() (helped by this answer).

This wasn't the whole story, as I got to this point and realised that the async process was hanging after a while due to the stream buffer filling up.

The key part that I needed (having set the streams to non-blocking previously) was to revert the streams to blocking mode, so that the buffer would drain into the receiving cat processes correctly.

To complete the code from my question:

// Iterate over the streams that are stil open
foreach(array_reverse($readables) as $stream) {
    // Revert the blocking mode
    stream_set_blocking($stream, true);
    $cmd = 'cat';

    // Receive input from an output stream for the previous process,
    // Send output into the internal unified output buffer
    $pipes = [
        0 => $stream,
        1 => $this->temp,
        2 => array("file", "/dev/null", 'w'),
    ];

    // Launch the process
    $this->cats[] = proc_open($cmd, $pipes, $outputPipes = []);
}