What happens if you run the following script?  Are there any error
messages?  Does the script finish after some time?

<?php
$ch = curl_init();
$fp = fopen('/dev/null', 'w');
for($i = 0; $i < 3; $i++) {
    curl_setopt($ch, CURLOPT_URL, 'http://127.0.0.1:9/');
    curl_setopt($ch, CURLOPT_FILE, $fp);
    curl_setopt($ch, CURLOPT_VERBOSE, true);
    curl_exec($ch);
}
?>

This is what I get:

root@Saguaro:/tmp # /opt/php72/bin/php
php*        php-cgi*    php-config* phpdbg*     phpize*     
root@Saguaro:/tmp # /opt/php72/bin/php !$
/opt/php72/bin/php test_script.php
* STATE: INIT => CONNECT handle 0x53113000; line 1605 (connection #-5000)
* Added connection 0. The cache now contains 1 members
*   Trying 127.0.0.1:9...
* TCP_NODELAY set
* STATE: CONNECT => WAITCONNECT handle 0x53113000; line 1661 (connection #0)
* connect to 127.0.0.1 port 9 failed: Operation timed out
* Failed to connect to 127.0.0.1 port 9: Operation timed out
* multi_done
* Closing connection 0
* The cache now contains 0 members
* STATE: INIT => CONNECT handle 0x53113000; line 1605 (connection #-5000)
* Added connection 1. The cache now contains 1 members
* Hostname 127.0.0.1 was found in DNS cache
*   Trying 127.0.0.1:9...
* TCP_NODELAY set
* STATE: CONNECT => WAITCONNECT handle 0x53113000; line 1661 (connection #1)
* connect to 127.0.0.1 port 9 failed: Operation timed out
* Failed to connect to 127.0.0.1 port 9: Operation timed out
* multi_done
* Closing connection 1
* The cache now contains 0 members
* STATE: INIT => CONNECT handle 0x53113000; line 1605 (connection #-5000)
* Added connection 2. The cache now contains 1 members
*   Trying 127.0.0.1:9...
* TCP_NODELAY set
* STATE: CONNECT => WAITCONNECT handle 0x53113000; line 1661 (connection #2)
* connect to 127.0.0.1 port 9 failed: Operation timed out
* Failed to connect to 127.0.0.1 port 9: Operation timed out
* multi_done
* Closing connection 2
* The cache now contains 0 members
root@Saguaro:/tmp # 

My php is now:

root@Saguaro:/tmp # /opt/php72/bin/php --version
PHP 7.2.27 (cli) (built: Jan 23 2020 19:30:35) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies

Which shows the very same behaviour of its predecessor.

Thanks for the fast reply!

While active support for PHP 7.2 has ended a while ago[1], this
doesn't appear related to the PHP version at all, but rather to

    Failed to connect to 127.0.0.1 port 9: Operation timed out

Apparently, your system does not support the discard protocol[2],
so the test should be skipped.

[1] <https://www.php.net/supported-versions.php>
[2] <https://tools.ietf.org/html/rfc863>

Yes I can confirm that I get the very same result with

PHP 7.3.14 (cli) (built: Jan 24 2020 13:26:08) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.14, Copyright (c) 1998-2018 Zend Technologies

and

PHP 7.4.2 (cli) (built: Jan 24 2020 16:08:23) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies

Thanks for testing with the newer PHP versions!

Could you please run the following script, and post its output
here?  I expect that to print int(28), but would like to have that
confirmed.

<?php
$ch = curl_init('http://127.0.0.1:9/');
curl_exec($ch);
var_dump(curl_error());
?>