Using curl_multi_select() in PHP to Wait for Active cURL Transfers

In PHP, using the cURL library for HTTP requests is very common. When handling multiple HTTP requests simultaneously, the curl_multi library can be used, and the curl_multi_select() function is often needed to wait for the current active cURL transfers to finish.

Function Definition

curl_multi_select(resource $mh[, float $timeout])

Parameters

$mh : the cURL multi handle to wait on.

$timeout : optional timeout in seconds; if set to 0 the function returns immediately.

Function Purpose

The curl_multi_select() function waits for the current active cURL transfers to complete and returns the number of handles ready for reading. Calling it keeps the script active during the waiting period.

Example

$mh = curl_multi_init();

$ch1 = curl_init();
curl_setopt($ch1, CURLOPT_URL, "https://example.com/api/request1");
curl_setopt($ch1, CURLOPT_RETURNTRANSFER, true);
curl_multi_add_handle($mh, $ch1);

$ch2 = curl_init();
curl_setopt($ch2, CURLOPT_URL, "https://example.com/api/request2");
curl_setopt($ch2, CURLOPT_RETURNTRANSFER, true);
curl_multi_add_handle($mh, $ch2);

do {
    $status = curl_multi_exec($mh, $active);
    if ($active) {
        curl_multi_select($mh); // wait for active transfers
    }
} while ($active && $status == CURLM_OK);

// After processing all requests, close handles
foreach ([$ch1, $ch2] as $ch) {
    curl_multi_remove_handle($mh, $ch);
    curl_close($ch);
}
curl_multi_close($mh);

The example creates a curl_multi handle, adds two individual curl handles, and uses a loop that calls curl_multi_exec() to process transfers. If there are still active transfers, curl_multi_select() is called to wait while allowing other tasks to run. The loop continues until all transfers are finished.

Finally, all curl handles and the multi handle are properly closed.

Summary

Using curl_multi_select() helps wait for active cURL transfers, keeping the script responsive and improving the efficiency and performance of multiple HTTP requests.

Note: Ensure proper error and exception handling when using this function to avoid infinite waiting.