So what you want to do is asynchronous execution of the cUrl Requests.

So you would need a asynchronous/parallel processing library for php.

pThreads

One of the prominent threading libraries for php is pthreads

You would need to first get the dll/so file and save it in the php/ext dir, and enable that extension in php.ini.

After That, this code would do your job:

class Request1 extends Thread {
    $username = 'xxxxx'; 
    $api_onfleet = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';    
    $url_onfleet = "https://onfleet.com/api/v2/tasks";
    public function run() {
        curl_setopt($curl, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);
        $request =  $this->url.'api/mail.send.json';

        // Generate curl request
        $session = curl_init($request);
        // Tell curl to use HTTP POST
        curl_setopt ($session, CURLOPT_POST, true);
       // Tell curl that this is the body of the POST
       curl_setopt ($session, CURLOPT_POSTFIELDS, $params);
       // Tell curl not to return headers, but do return the response
       curl_setopt($session, CURLOPT_HEADER, false);
       curl_setopt($session, CURLOPT_RETURNTRANSFER, true);

        // obtain response
        $response = curl_exec($session);
        curl_close($session);
    }
}


class Request2 extends Thread {
    $username = 'xxxxx'; 
    $api_onfleet = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';    
    $url_onfleet = "https://onfleet.com/api/v2/tasks";
    public function run() {
      $ch = curl_init();
      curl_setopt($ch, CURLOPT_URL, $url_onfleet);
      curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
      curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
      curl_setopt($ch, CURLOPT_USERPWD, $this->api_onfleet);
      curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
      curl_setopt($ch, CURLOPT_ENCODING, "");  
      curl_setopt($ch, CURLOPT_POST, true);
      curl_setopt($ch, CURLOPT_POSTFIELDS, '{"destination":{"address":{"unparsed":"'.$pickup_address.'"},"notes":"'.$comments.'"},"recipients":[{"name":"'.$name.'","phone":"+61'.$phone.'","notes":"Number of riders: '.$riders.'"}],"completeBefore":'.$timestamp.',"pickupTask":"yes","autoAssign":{"mode":"distance"}}');

      $result_pickup = curl_exec($ch);
      curl_close($ch);

      // Post the Dropoff task to Onfleet
      $curl = curl_init();
      curl_setopt($curl, CURLOPT_URL, $this->url_onfleet);
      curl_setopt($curl, CURLOPT_FOLLOWLOCATION, 1);
      curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
      curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
      curl_setopt($curl, CURLOPT_USERPWD, $this->api_onfleet);
      curl_setopt($curl, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
      curl_setopt($curl, CURLOPT_ENCODING, "");  
      curl_setopt($curl, CURLOPT_POST, true);
      curl_setopt($curl, CURLOPT_POSTFIELDS, '{"destination":{"address":{"unparsed":"'.$dropoff_address.'"},"notes":"'.$comments.'"},"recipients":[{"name":"'.$name.'","phone":"+61'.$phone.'","notes":"Number of riders: '.$riders.'"}],"autoAssign":{"mode":"distance"}}');

      $result_dropoff = curl_exec($curl);
      curl_close($curl);
    }
}

$req1 = new Request1();
$req1->start();
$req2 = new Request2();
$req2->start();

So, basically you need to create a class which extends the Thread class and everything you want to run asynchronously (rather parallely), would be put in the function run() of the class.

When you want to start the thread just instantiate the class in a variable, and call the start method of the object, like $threadsObject->start() and everything in the run() would be executed on another thread.

Reference:

1. class::Thread
2. Thread::start

That's it.

Async cURL

The other way is to use the built in asynchronous cURL functions.

So, when using curl_multi_*, your code would look something like:

$username = 'xxxxx'; 
$api_onfleet = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';    
$url_onfleet = "https://onfleet.com/api/v2/tasks";

curl_setopt($curl, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);
    $request =  $url.'api/mail.send.json';

    // Generate curl request
    $session = curl_init($request);
    // Tell curl to use HTTP POST
    curl_setopt ($session, CURLOPT_POST, true);
    // Tell curl that this is the body of the POST
    curl_setopt ($session, CURLOPT_POSTFIELDS, $params);
    // Tell curl not to return headers, but do return the response
    curl_setopt($session, CURLOPT_HEADER, false);
    curl_setopt($session, CURLOPT_RETURNTRANSFER, true);


     // Post the Pickup task to Onfleet
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url_onfleet);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_USERPWD, $api_onfleet);
    curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_setopt($ch, CURLOPT_ENCODING, "");  
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, '{"destination":{"address":{"unparsed":"'.$pickup_address.'"},"notes":"'.$comments.'"},"recipients":[{"name":"'.$name.'","phone":"+61'.$phone.'","notes":"Number of riders: '.$riders.'"}],"completeBefore":'.$timestamp.',"pickupTask":"yes","autoAssign":{"mode":"distance"}}');
$mh = curl_multi_init();
curl_multi_add_handle($mh,$session);
curl_multi_add_handle($mh,$ch);

$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);
    }
}
//close the handles
curl_multi_remove_handle($mh, $ch1);
curl_multi_remove_handle($mh, $ch2);
curl_multi_close($mh);

Suggested Reading:

1. curl_multi_init()
2. curl_multi_exec()
3. curl_multi_add_handle()
4. curl_multi_remove_handle()

You are right that they fill the same purpose. Though, the SoapClient has a lot of functionality for supporting SOAP requests built in and is using Curl somewhere as well.

As you can see in the discussion on SOAP with Curl the programmer constructs the SOAP envelope, the SoapClient can do that for you. The same holds for doing the method calls.

In the end, it is much easier to use the SoapClient.

CURLOPT_POSTFIELDS as the name suggests, is for the body (payload) of a POST request. For GET requests, the payload is part of the URL in the form of a query string.

In your case, you need to construct the URL with the arguments you need to send (if any), and remove the other options to cURL.

curl_setopt($ch, CURLOPT_URL, $this->service_url.'user/'.$id_user);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_HEADER, 0);

//$body = '{}';
//curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "GET"); 
//curl_setopt($ch, CURLOPT_POSTFIELDS,$body);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);