There is a server which simply waits 2 seconds and then outputs Hello World. The client-parallel script triggers the curl request, then sleeps for a second (representing some amount of work) and then processes the response. The client-linear request triggers the curl request and immediately waits for the answer and processes it, then it continue…

client-linear.php

<?php
$ch1 = curl_init();

// set URL and other appropriate options
curl_setopt($ch1, CURLOPT_URL, "http://localhost/~naderman/sleep.php");
curl_setopt($ch1, CURLOPT_HEADER, 0);
curl_setopt($ch1, CURLOPT_RETURNTRANSFER, true);

//create the multiple cURL handle
$mh = curl_multi_init();

//add the two handles
curl_multi_add_handle($mh,$ch1);

$active = null;
//execute the handles
do {
    $mrc = curl_multi_exec($mh, $active);
} while ($mrc == CURLM_CALL_MULTI_PERFORM);

while ($active && $mrc == CURLM_OK) {
    if (curl_multi_select($mh) != -1) {
        do {
            $mrc = curl_multi_exec($mh, $active);
        } while ($mrc == CURLM_CALL_MULTI_PERFORM);
    }
}

// Simulated workload: sleep after the request is handled
sleep(1);

//close the handles
curl_multi_remove_handle($mh, $ch1);
curl_multi_close($mh);

client-parallel.php

<?php
$ch1 = curl_init();

// set URL and other appropriate options
curl_setopt($ch1, CURLOPT_URL, "http://localhost/~naderman/sleep.php");
curl_setopt($ch1, CURLOPT_HEADER, 0);
curl_setopt($ch1, CURLOPT_RETURNTRANSFER, true);

//create the multiple cURL handle
$mh = curl_multi_init();

//add the two handles
curl_multi_add_handle($mh,$ch1);

$active = null;
//execute the handles
do {
    $mrc = curl_multi_exec($mh, $active);
} while ($mrc == CURLM_CALL_MULTI_PERFORM);

// Simulated workload: sleep while request is handled
sleep(1);

while ($active && $mrc == CURLM_OK) {
    if (curl_multi_select($mh) != -1) {
        do {
            $mrc = curl_multi_exec($mh, $active);
        } while ($mrc == CURLM_CALL_MULTI_PERFORM);
    }
}

//close the handles
curl_multi_remove_handle($mh, $ch1);
curl_multi_close($mh);

results

naderman@Montsoreau:~/tmp$ time php -f client-parallel.php

real    0m2.029s
user    0m0.012s
sys     0m0.012s
naderman@Montsoreau:~/tmp$ time php -f client-linear.php

real    0m3.029s
user    0m0.012s
sys     0m0.012s

sleep.php

<?php

sleep(2);

echo "Hello World!\n";