curl error 18 - transfer closed with outstanding read data remaining

Error 18 means that the transfer was closed with outstanding read data remaining. This can be caused by a variety of issues, including:

• Network issues (e.g. a dropped connection)
• A problem with the server you are trying to connect to
• A problem with the client you are using (e.g. curl)

To troubleshoot this issue in PHP, you can try the following:

• Check the network connection and ensure that it is stable
• Check the server you are trying to connect to and see if there are any known issues
• Increase CURLOPT_TIMEOUT and CURLOPT_CONNECTTIMEOUT to prevent premature connection drops
• Enable CURLOPT_VERBOSE => true to log detailed transfer information and identify where the connection fails
• If you are using curl, try using a different version or using a different client to see if the problem persists

If the problem persists after trying these steps, you may need to seek additional assistance from your network administrator or the server administrator.

PHP cURL Configuration Example

The following example demonstrates how to configure cURL in PHP to handle timeouts properly and enable verbose debugging:

$ch = curl_init('https://example.com/api/data');
curl_setopt_array($ch, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_TIMEOUT => 30,
    CURLOPT_CONNECTTIMEOUT => 10,
    CURLOPT_VERBOSE => true, // Outputs debug info to STDERR
    CURLOPT_SSL_VERIFYPEER => true,
]);

$response = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
$curlError = curl_error($ch);

if ($curlError) {
    echo "cURL Error (" . curl_errno($ch) . "): " . $curlError;
} else {
    echo "Response: " . $response;
}

curl_close($ch);