Can't connect to HTTPS site using cURL. Returns 0 length content instead. What can I do?

There are several possible reasons why cURL is returning a 0 length response when trying to connect to an HTTPS site. Some potential issues include:

1. SSL certificate issues: Make sure the SSL certificate for the site you're trying to connect to is valid and up-to-date. For self-signed certificates, you must set CURLOPT_SSL_VERIFYPEER and CURLOPT_SSL_VERIFYHOST to false. Setting them to true will reject self-signed certificates.
2. Firewall or proxy settings: Check your firewall and proxy settings to ensure that they are not blocking cURL from connecting to the site.
3. cURL version: Make sure that you are using the latest version of cURL, as older versions may have issues connecting to certain sites.
4. Missing CURLOPT_RETURNTRANSFER: The most common cause of a 0-length response is omitting this option. Without it, cURL prints the response directly to the output instead of returning it to a variable.
5. curl_setopt(): use CURLOPT_CAINFO to specify the path to the CA cert bundle file.
6. Check the URL of the site you're trying to connect to. Make sure that it's typed correctly and that you're using the correct protocol (HTTP or HTTPS).

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://example.com");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); // Captures output instead of printing it
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false); // Set to true in production with a valid CA bundle
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
// curl_setopt($ch, CURLOPT_CAINFO, '/path/to/ca-bundle.crt');

$response = curl_exec($ch);
curl_close($ch);

if ($response === false) {
    echo 'cURL Error: ' . curl_error($ch);
} else {
    echo "Response Length: " . strlen($response) . "\n";
    echo $response;
}

Try these steps and see if any of them resolve the issue. If the problem persists, please provide more details about your specific setup and the error messages you're receiving.