<?php

$cmd = "php /path/to/b.php";

$descriptorspec = [
   0 => ["pipe", "r"],   // stdin is a pipe that the child will read from
   1 => ["pipe", "w"],   // stdout is a pipe that the child will write to
   2 => ["pipe", "w"]    // stderr is a pipe that the child will write to
];

$process = proc_open($cmd, $descriptorspec, $pipes, realpath('./'), []);

// your program here
echo 'this is a.php stuff';
for($i = 0; $i < 5; $i++) { echo "."; sleep(1); }

// read the output of b.php at the end
if (is_resource($process)) {
    while ($s = fgets($pipes[1])) {
        echo $s;
    }
}