proc_open is phar doesn't work?

hello,
I have problem using proc_open in phar file.
here is sample code

    <?php
    $p = new Phar('test.phar', FilesystemIterator::CURRENT_AS_FILEINFO |
FilesystemIterator::KEY_AS_FILENAME);
    $p->startBuffering();
    $stub = <<<STR
    #!/usr/bin/env php
    <?php
    Phar::mapPhar();
    include "phar://test.phar/parent.php";
    __HALT_COMPILER();
    STR;
    $p->setStub($stub);
    $parent = <<<STR
    <?php
    \$descriptorspec = array(
       0 => array("pipe", "r"),
       1 => array("pipe", "w"),
       2 => array("pipe", "w")
    );
    \$path = realpath(__DIR__ . '/child.php');
    \$process = proc_open('php ' . \$path, \$descriptorspec, \$pipes);
    if (is_resource(\$process)) {
        fclose(\$pipes[0]);
        \$stdout = stream_get_contents(\$pipes[1]);
        fclose(\$pipes[1]);
        fclose(\$pipes[2]);
        \$return_value = proc_close(\$process);
        var_dump(\$stdout);
        var_dump(\$return_value);
    }
    STR;
    $p['parent.php'] = $parent;
    $child = <<<STR
    <?php
    file_put_contents(sys_get_temp_dir() . '/child', "child");
    echo "child";
    STR;
    $p['child.php'] = $child;
    $p->stopBuffering();

I expect /tmp/child file is created by child.php and 1 is set to $stdout as
status code.
but actually child.php seems not to be executed and I get 0 as exit code.

as long as reading php-src and debugging, I find php tries to execl 'php
phar://test.phar/child.php' at here

https://github.com/php/php-src/blob/0ca4358ade18c3839414580da35660c6e9be08d5/ext/standard/proc_open.c#L842

of course it shouldn't work because php cannot execute phar://...
independently.
so does proc_open in phar never work?

I'm not sure although I find some evidences. I cannot believe no one has
not tried to use proc_open in phar until now...
according to git history, this code has not been changed at least 2 years.

is my understanding correct? if so, is there any workaround?