<?php
/* 1. URLs a consultar */
$urls = [
    'https://httpbin.org/delay/2',
    'https://httpbin.org/uuid',
    'https://httpbin.org/headers'
];

/* 2. Crear el “multi handle” */
$mh = curl_multi_init();

/* 3. Crear un handle individual para cada URL y añadirlo al multi */
$handles = [];
foreach ($urls as $i => $url) {
    $ch = curl_init($url);
    curl_setopt_array($ch, [
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_FOLLOWLOCATION => true,
        CURLOPT_TIMEOUT        => 10,
        CURLOPT_HTTPHEADER     => ['Accept: application/json']
    ]);
    curl_multi_add_handle($mh, $ch);
    $handles[$i] = $ch;   // guardamos referencia para después
}

/* 4. Ejecutar todas las transferencias en paralelo */
$running = 0;
do {
    curl_multi_exec($mh, $running);
    if ($running) {
        // esperar activamente hasta que haya actividad (evita busy-wait)
        curl_multi_select($mh);
    }
} while ($running > 0);

/* 5. Recoger respuestas y cerrar */
$responses = [];
foreach ($handles as $i => $ch) {
    if (curl_errno($ch) === 0) {
        $responses[$i] = [
            'url'  => $urls[$i],
            'code' => curl_getinfo($ch, CURLINFO_HTTP_CODE),
            'body' => curl_multi_getcontent($ch)
        ];
    } else {
        $responses[$i] = [
            'url'   => $urls[$i],
            'error' => curl_error($ch)
        ];
    }
    curl_multi_remove_handle($mh, $ch);
    curl_close($ch);
}
curl_multi_close($mh);

/* 6. Ya tienes las tres respuestas, puedes devolverlas al cliente */
header('Content-Type: application/json');
echo json_encode($responses, JSON_PRETTY_PRINT);

/* ó */

$> composer require guzzlehttp/guzzle:^7.0

use GuzzleHttp\Client;
use GuzzleHttp\Promise;

$client = new Client(['timeout' => 10]);

$promises = [
    $client->getAsync('https://httpbin.org/delay/2'),
    $client->getAsync('https://httpbin.org/uuid'),
    $client->getAsync('https://httpbin.org/headers')
];

$responses = Promise\Utils::settle($promises)->wait(); // espera a todas

foreach ($responses as $r) {
    echo $r['value']->getBody();
}