$ cat /tmp/test.php
<?php
      $client = new \http\Client;

$ php /tmp/test.php
PHP Fatal error:  Uncaught exception 'http\Exception\UnexpectedValueException' with message 'Failed to locate "(null)" client request handler' in /tmp/test.php:2
Stack trace:
#0 /tmp/test.php(2): http\Client->__construct()
#1 {main}
  thrown in /tmp/test.php on line 2

$ php -v
PHP 5.6.40-53+ubuntu18.04.1+deb.sury.org+1 (cli)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2016, by Zend Technologies

$ dpkg --get-selections | egrep 'php|curl'
libapache2-mod-php5.6                           install
libcurl3:amd64                                  install
libcurl3-gnutls:amd64                           install
php-common                                      install
php-pear                                        install
php5.6                                          install
php5.6-cli                                      install
php5.6-common                                   install
php5.6-dev                                      install
php5.6-http                                     install
php5.6-json                                     install
php5.6-mbstring                                 install
php5.6-mysql                                    install
php5.6-opcache                                  install
php5.6-propro                                   install
php5.6-raphf                                    install
php5.6-readline                                 install
php5.6-xml                                      install
pkg-php-tools                                   install