Using curl_multi_exec() in PHP to Execute Multiple cURL Requests Concurrently

cURL is an open‑source library for network communication in PHP, supporting protocols such as HTTP, HTTPS, and FTP.

The curl_multi_exec() function allows simultaneous execution of multiple cURL handles, improving request efficiency.

Function Prototype

int curl_multi_exec(resource $multi_handle, int &$still_running)

Parameters

$multi_handle

: the multi‑handle resource created by curl_multi_init(). $still_running: variable that receives the number of handles still running.

Return Value

If an error occurs, curl_multi_exec() returns an error code; otherwise it returns 0 on success.

Example Code

<?php
// Create a multi‑handle
$multi_handle = curl_multi_init();

// Create individual handles and add them to the multi‑handle
$handles = array();
$handles[] = create_curl_handle("https://example.com/api1");
$handles[] = create_curl_handle("https://example.com/api2");
$handles[] = create_curl_handle("https://example.com/api3");

foreach ($handles as $handle) {
    curl_multi_add_handle($multi_handle, $handle);
}

// Execute the handles
$running = null;
do {
    curl_multi_exec($multi_handle, $running);
    curl_multi_select($multi_handle); // wait for I/O
} while ($running > 0);

// Retrieve responses
foreach ($handles as $handle) {
    $response = curl_multi_getcontent($handle);
    echo $response;
    curl_multi_remove_handle($multi_handle, $handle);
    curl_close($handle);
}

// Close the multi‑handle
curl_multi_close($multi_handle);

function create_curl_handle($url) {
    $handle = curl_init();
    curl_setopt($handle, CURLOPT_URL, $url);
    curl_setopt($handle, CURLOPT_RETURNTRANSFER, true);
    return $handle;
}
?>

Important Notes

Call curl_multi_add_handle() for each session before invoking curl_multi_exec().

Ensure all requests have finished (running handles equal 0) before using curl_multi_getcontent().

Remove each handle with curl_multi_remove_handle() and close it with curl_close() after processing.

Conclusion

The curl_multi_exec() function is a powerful tool for concurrently executing multiple cURL sessions, boosting request efficiency; the provided example demonstrates its usage and can be adapted for various backend development scenarios.