How to clean up the connection pool with curl_close after the cURL request is over?

It is a common way to use cURL (Client URL Library) in PHP for network requests. Whether it is sending a GET request to crawl the page content or submitting data through a POST request, cURL provides a rich and flexible functional interface. When using these functions, timely closing and cleaning up connection resources is a key link in ensuring program performance and stability. This article will focus on how to correctly close the handle and free the resource using the curl_close function after the request is completed.

1. Introduction to cURL

cURL is a powerful tool for interacting with servers in PHP scripts for HTTP, HTTPS, FTP and other protocols. By initializing a cURL session, setting various options, executing requests and getting return results, developers can implement various complex network communication operations.

2. Why use curl_close?

Each cURL handle created through curl_init is allocated certain system resources. If curl_close is not called to close this handle after the request is completed, these resources will not be released, causing memory leaks or connection pool exhaustion.

In high concurrency scenarios, this problem is even more serious and may lead to service crashes. Therefore, whether the request succeeds or fails, be sure to call curl_close after completion to destroy the handle and free the resource .

3. Basic examples of using curl_close

Here is a typical cURL request process that shows how to use curl_close correctly:

 <?php

// initialization cURL Session
$ch = curl_init();

// Setting target URL
curl_setopt($ch, CURLOPT_URL, "https://gitbox.net/api/data");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

// Execute a request
$response = curl_exec($ch);

// Check if the request is wrong
if (curl_errno($ch)) {
    echo 'Request error: ' . curl_error($ch);
} else {
    echo 'Response content: ' . $response;
}

// closureSession并释放资源
curl_close($ch);
?>

In the above example, curl_close($ch) is the last step in the entire request process, ensuring that the program does not retain unnecessary connections.

4. The relationship between connection multiplexing and curl_close

Modern PHP-cURL supports Connection Reuse, especially in scenarios where curl_multi interface or HTTP/2 multiplexing is enabled. If you enable the persistent connection option, for example:

 curl_setopt($ch, CURLOPT_FORBID_REUSE, false);

At this time, the connection may be placed in the connection pool and wait for reuse. But please note that the management of a connection pool is not the same as the management of handles . Even if the underlying connection is reused, curl_close is still an operation that must be performed, and it is responsible for destroying the data structures related to the handle in the user space.

5. Things to note when cooperating with curl_multi

If you are using curl_multi_init multi-handle concurrent request method, you also need to close all handles in turn after the operation is completed:

 $mh = curl_multi_init();
$ch1 = curl_init("https://gitbox.net/api/one");
$ch2 = curl_init("https://gitbox.net/api/two");

curl_setopt($ch1, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch2, CURLOPT_RETURNTRANSFER, true);

curl_multi_add_handle($mh, $ch1);
curl_multi_add_handle($mh, $ch2);

// Perform concurrent requests
do {
    $status = curl_multi_exec($mh, $active);
    curl_multi_select($mh);
} while ($active && $status == CURLM_OK);

// Get results
$response1 = curl_multi_getcontent($ch1);
$response2 = curl_multi_getcontent($ch2);

// Remove and close the handle
curl_multi_remove_handle($mh, $ch1);
curl_multi_remove_handle($mh, $ch2);
curl_close($ch1);
curl_close($ch2);

// closure multi Handle
curl_multi_close($mh);

It is particularly emphasized here: make sure that all operations have been completed before removing the handle, otherwise undefined behavior may occur.

6. Summary

When using cURL, calling curl_close in time to release resources is a basic requirement for writing robust PHP applications. While modern versions of PHP and operating systems can automatically recycle resources at the end of a process, relying on system recycling is an unsafe practice. Actively closing resources can not only improve the stability of the program, but also help locate problems and optimize performance.

In the case of high-frequency requests, concurrent execution of tasks, and calling external APIs, developers should include curl_close as an unomitted step into the standard process.

Through good resource management habits, we can avoid many hidden problems and make PHP applications more robust and efficient.