PHP cURL curl_multi_select() Function

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The PHP Client URL curl_multi_select() function is used in together with curl_multi_exec() to make multiple cURL requests at once. This method allows a PHP code to wait until activity is detected on any of the cURL handles in a curl_multi session.

Syntax

Below is the syntax of the PHP cURL curl_multi_select() function −

int curl_multi_select ( resource $mh [, float $timeout = 1.0] )

Parameters

Following are the parameters required for the curl_multi_exec() function −

• $mh − It is the cURL multi handle (resource) returned by curl_multi_init().

• $timeout − It is the time in seconds to wait for a response.

Return Value

The curl_multi_select() function returns an integer −

• -1: Indicates that an error has occurred.
• 0: Indicates that there has been no activity all over the set timeout period.
• >0: Indicates the number of active cURL handles that have been found. This shows that there is one or more active cURL handles.

PHP Version

The curl_multi_select() function was introduced in core PHP 5 and is compatible with PHP 7, and PHP 8.

Example 1

Here is the basic example of how to use the PHP cURL curl_multi_select() function to check for activity on any of the handles and blocks to determine either activity or a timeout.

<?php
   // Start the multi handle
   $mh = curl_multi_init();

   // Add individual cURL handles
   $ch1 = curl_init('https://abc.com/users/1');
   $ch2 = curl_init('https://abc.com/albums/1');
   curl_multi_add_handle($mh, $ch1);
   curl_multi_add_handle($mh, $ch2);

   $active = null;

   // Execute the handles
   do {
      $mrc = curl_multi_exec($mh, $active);
      if (curl_multi_select($mh) === -1) {
         usleep(100);
      }
   } while ($mrc == CURLM_CALL_MULTI_PERFORM || $active);

   // Close the handles
   curl_multi_remove_handle($mh, $ch1);
   curl_multi_remove_handle($mh, $ch2);
   curl_multi_close($mh);
?> 

Output

Here is the outcome of the following code −

{ "id": 1, "name": "Leanne Graham", "username": "Bret", "email": "Sincere@april.biz", "address": { "street": "Kulas Light", "suite": "Apt. 556", "city": "Gwenborough", "zipcode": "92998-3874", "geo": { "lat": "-37.3159", "lng": "81.1496" } }, "phone": "1-770-736-8031 x56442", "website": "hildegard.org", "company": { "name": "Romaguera-Crona", "catchPhrase": "Multi-layered client-server neural-net", "bs": "harness real-time e-markets" } }
{ "userId": 1, "id": 1, "title": "quidem molestiae enim" }

Example 2

In the below PHP code we will use the curl_multi_select() function to handle many cURL requests which stops the script from executing until any of the cURL handles that the multi-handle ($mh) is active.

<?php
   // Initialize multi-handle
   $mh = curl_multi_init();
   
   // Image URLs to fetch
   $imageUrls = [
       'http://example.com/image1.jpg',
       'http://example.com/image2.jpg',
       'http://example.com/image3.jpg'
   ];
   
   // Initialize cURL handles and add to multi-handle
   foreach ($imageUrls as $url) {
       $ch = curl_init($url);
       curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); // Set return transfer to true
       curl_multi_add_handle($mh, $ch);
   }
   
   // Execute requests and wait for activity
   $active = null;
   do {
       $mrc = curl_multi_exec($mh, $active);
       if (curl_multi_select($mh) === -1) {
           usleep(100); // Sleep for a short time if select returns -1
       }
   } while ($mrc == CURLM_CALL_MULTI_PERFORM || $active);
   
   // Process responses (in this case, download images)
   foreach ($imageUrls as $url) {
       $ch = curl_init($url);
       $fileName = basename($url);
       $file = fopen($fileName, 'wb');
       curl_setopt($ch, CURLOPT_FILE, $file); // Write response directly to file
       curl_exec($ch);
       curl_close($ch);
       fclose($file);
   }
   
   // Close multi-handle
   curl_multi_close($mh);

   echo "The images has been downloaded.";

Output

This will generate the below output −

The images has been downloaded.

Example 3

Now we will use the curl_multi_select() function to make concurrent API requests.

<?php
   // Initialize multi-handle
   $mh = curl_multi_init();

   // API endpoints with parameters
   $endpoints = [
      'https://jsonplaceholder.typicode.com/todos/?id=3',
      'https://jsonplaceholder.typicode.com/todos/?id=7',
      'https://jsonplaceholder.typicode.com/todos/?id=5'
   ];

   // Initialize cURL handles and add to multi-handle
   foreach ($endpoints as $endpoint) {
      $ch = curl_init($endpoint);

      // Set return transfer to true
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); 
      curl_multi_add_handle($mh, $ch);
   }

   // Execute requests and wait for activity
   $active = null;
   do {
      $mrc = curl_multi_exec($mh, $active);
      if (curl_multi_select($mh) === -1) {
         usleep(100); // Sleep for a short time if select returns -1
      }
   } while ($mrc == CURLM_CALL_MULTI_PERFORM || $active);

   // Process responses
   foreach ($endpoints as $endpoint) {
      $ch = curl_init($endpoint);
      $response = curl_exec($ch); // Execute request
      echo "Response from " . $endpoint . ": " . substr($response, 0, 100) . "...<br>"; /
      curl_close($ch);
   }

   // Close multi-handle
   curl_multi_close($mh);
?> 

Output

This will create the below output −

[ { "userId": 1, "id": 3, "title": "fugiat veniam minus", "completed": false } ]Response from https://jsonplaceholder.typicode.com/todos/?id=3: 1...
[ { "userId": 1, "id": 7, "title": "illo expedita consequatur quia in", "completed": false } ]Response from https://jsonplaceholder.typicode.com/todos/?id=7: 1...
[ { "userId": 1, "id": 5, "title": "laboriosam mollitia et enim quasi adipisci quia provident illum", "completed": false } ]Response from https://jsonplaceholder.typicode.com/todos/?id=5: 1...

Summary

The curl_multi_select() is a built-in function to make multiple URL requests at once. If curl_multi_select() is executed successfully, your PHP code will handle multiple cURL requests more effectively and efficiently, with less resources and improving speed for concurrent HTTP activity.