[php-src] Issue #14915: curl_error() is missing a fallback as expected and documented in curl manual

Issue: https://github.com/php/php-src/issues/14915 Author: arekm ### Description The following code: ```php $session = curl_init(); curl_setopt($session, CURLOPT_URL, "https://someserver/cmd.php"); curl_setopt($session, CURLOPT_USERPWD, "user:pass"); curl_setopt($session, CURLOPT_SSL_VERIFYHOST, 1); curl_setopt($session, CURLOPT_RETURNTRANSFER, 1); curl_setopt($session, CURLOPT_TIMEOUT, 70); curl_setopt($session, CURLOPT_CONNECTTIMEOUT, 10); curl_setopt($session, CURLOPT_SSL_VERIFYPEER, FALSE); curl_setopt($session, CURLOPT_HTTPHEADER, array('Content-type: application/json', 'Expect:')); curl_setopt($session, CURLOPT_FAILONERROR, 1); curl_setopt($session, CURLOPT_POST, 1); curl_setopt($session, CURLOPT_POSTFIELDS, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"); $response = curl_exec($session); echo "curl_error: " . curl_error($session) . ", curl_errno: " . curl_errno($session) . "\n"; ``` Resulted in this output: ``` $ php83 --version PHP 8.3.9 (cli) (built: Jul 7 2024 09:48:02) (NTS) Copyright (c) The PHP Group Zend Engine v4.3.9, Copyright (c) Zend Technologies $ php83 test.php curl_error: , curl_errno: 16 ``` But I expected this output instead: ``` curl_error: Error in the HTTP2 framing layer, curl_errno: 16 ``` since that's what curl cmdline returns ``` $ sh test.sh curl: (16) Error in the HTTP2 framing layer ``` Server is some old lighttpd 1.4.73 which most likely mishandles HTTP/2 and allows me to reproduce such error but unfortunately it's not reachable over internet. But I have an explanation on what's wrong. php is currently doing ``` PHP_FUNCTION(curl_error) { [...] if (ch->err.no) { ch->err.str[CURL_ERROR_SIZE] = 0; RETURN_STRING(ch->err.str); } else { RETURN_EMPTY_STRING(); } } ``` and err.str is set ``` curl_easy_setopt(ch->cp, CURLOPT_ERRORBUFFER, ch->err.str); ``` **The problem** curl documentation says that errobuffer sometimes can be not filled with error and then you are expected to fallback to curl_easy_strerror(). See example C code in manual: https://curl.se/libcurl/c/CURLOPT_ERRORBUFFER.html That's what curl cmdline does: https://github.com/curl/curl/blob/curl-8_8_0/src/tool_operate.c#L402-L403 (msg is their CURLOPT_ERRORBUFFER buffer) Similar bug existed in pycurl until got fixed in latest release: https://github.com/pycurl/pycurl/commit/88d2bfff565bf62504a812beb7d07f7f553bdcfa **Solution** Solution is to return curl_easy_strerror() information instead of RETURN_EMPTY_STRING() ### PHP Version PHP 8.3.9 (all existing php versions including git master) ### Operating System _No response_