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When downloading files or fetching online resources using PHP, cURL is one of the most commonly used tools. Generally, we use curl_exec() to execute requests and wait synchronously for them to complete. However, in certain advanced situations, such as when you need to pause or wait, the curl_pause() function is required.
This article will provide an in-depth explanation of the role of curl_pause(), its use cases, and how to combine it with other cURL functions to achieve more complex download control.
curl_pause() is a function provided by libcurl to control ongoing transfer operations, allowing you to pause or resume receiving (reading) or sending (writing) data.
In PHP, the function prototype is as follows:
curl_pause(CurlHandle $handle, int $bitmask): int
The $bitmask parameter can be a combination of the following constants:
CURLPAUSE_RECV: pause receiving data
CURLPAUSE_SEND: pause sending data
CURLPAUSE_ALL: pause both sending and receiving
CURLPAUSE_CONT: continue transfer (resume)
The return value is a CURLcode status code, usually 0 indicating success.
You might need to dynamically pause a download based on bandwidth, user actions, or server responses. For example, when bandwidth is limited, you can pause a download task and resume it later.
$ch = curl_init("https://example.com/largefile.zip");
$fp = fopen("largefile.zip", "w");
<p>curl_setopt($ch, CURLOPT_FILE, $fp);<br>
curl_setopt($ch, CURLOPT_WRITEFUNCTION, function($ch, $data) use (&$paused) {<br>
static $total = 0;<br>
$len = </span>strlen(</span>$data);<br>
</span>$total += </span>$len;</p>
if (</span>$total >= </span>1024 * </span>1024 && !</span>$paused) {
</span>curl_pause($ch, CURLPAUSE_RECV);
</span>$paused = true;
}
</span>return $len;
});
curl_exec($ch);
// Resume later
if ($paused) {
sleep(5); // Assume waiting 5 seconds
curl_pause($ch, CURLPAUSE_CONT);
curl_exec($ch);
}
curl_close($ch);
fclose($fp);
When handling multiple concurrent requests using curl_multi_exec(), curl_pause() becomes even more important. You can precisely pause a specific handle without affecting others.
$mh = curl_multi_init();
</span>$chs = [];
<p></span>foreach ($urls as $url) {<br>
$ch = </span>curl_init($url);<br>
</span>curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);<br>
</span>curl_multi_add_handle($mh, $ch);<br>
</span>$chs[] = $ch;<br>
}</p>
<p>// Pause a request under certain conditions<br>
curl_pause($chs[0], CURLPAUSE_RECV);</p>
<p>// Resume:<br>
curl_pause($chs[0], CURLPAUSE_CONT);<br>
Not all servers or transfers support pause and resume. Especially on servers without Range support, pausing might be equivalent to interrupting the transfer.
If you use CURLOPT_RETURNTRANSFER, you cannot retrieve remaining data with curl_exec() after pausing. You need to use curl_multi or stream handling instead.
Data may still be buffered after pausing, so you might still see the WRITEFUNCTION callback invoked once.
curl_pause() offers PHP finer control over HTTP transfer behavior and is suitable for advanced scenarios that require dynamic control of data flow. It shows great value in file downloads, streaming media processing, and multi-threaded task management.
When using it, combine WRITEFUNCTION callbacks with the curl_multi environment to manage transfer states flexibly. Mastering this function can give your network transfer logic unprecedented flexibility and control.
