Description:
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Some FTP function (ftp_nlist, ftp_put, ..etc) show error message "failed: Operation now in progress (115)"
when failed to establish FTP data connection.
This error message comes from EINPROGRESS,
and EINPROGRESS comes from connect because sockfd is non-blocking.
https://github.com/php/php-src/blob/PHP-7.3.4/main/network.c#L308
Therefore, this error message not show the root cause of connection failure.

I think result from SO_ERROR should be used for displaying error message.
https://github.com/php/php-src/blob/PHP-7.3.4/main/network.c#L352

Test script:
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<?php
// Assume that there is a FTP server on localhost whose port for control connection(21) is reachable, but ports for data connection is unreachable.
// eg.
// sudo docker run -d -e FTP_USER_PASS=pass -e FTP_USER_NAME=test -e FTP_USER_HOME=/home/test --name ftpd_server -p 21:21 -e "PUBLICHOST=localhost" --rm stilliard/pure-ftpd:hardened
$c = ftp_connect('localhost', 21);
ftp_login($c, 'test', 'pass');
ftp_pasv($c, true);
ftp_nlist($c, './');

Expected result:
----------------
more appropriate error message is displayed.

Actual result:
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Warning: ftp_nlist(): php_connect_nonb() failed: Operation now in progress (115) in /***/test.php on line 8