This certainly makes sense. But does this also cover the use-case of me downloading the installer phar file and trying to run it without curl? Or is that already handled.

Is curl really needed anyway ? (I mean AFAIK, guzzle does not rely only on curl but has many different handlers available)
Maybe, instead of requiring curl, we could add a check through this function or similar in order to ensure one of the needed handler exist in order to make the request, or throw an appropriated exception in case it isn't.

Is the cURL extension available by default on Windows?

I tested this. cURL is not required, to my understanding.

I ran everything using docker with this dockerfile:

FROM debian:jessie
VOLUME .:/root/shared
WORKDIR /root/shared
RUN apt-get update && apt-get install -y php5-cli

ENTRYPOINT /bin/bash

i then built and ran it:

docker build -t symfony-test .
docker run -it -v `pwd`:/root/shared symfony-test /bin/bash

Out-of the box, the symfony installer worked on the environment. I double-checked to make sure that curl was not installed.

When I set allow_url_fopen = Off in php.ini i got this error instead, when i ran the installer:

So (as it is implemented right now) guzzle in the symfony installer tries curl first and then fopen (as can be seen here: https://github.com/guzzle/guzzle/blob/5.3/src/Utils.php#L181 ).

So, to summarize: Curl is not required and you will get an error message if you lack curl and the allow_url_open-flag.

I wrote this (to replace this line: https://github.com/symfony/symfony-installer/blob/master/src/Symfony/Installer/DownloadCommand.php#L224 ) if we want to make the error message more "symfony" and a bit less confusing for new users:

        try {
            $handler = \GuzzleHttp\Utils::getDefaultHandler();
        } catch (\RuntimeException $e) {
            throw new \RuntimeException(
                'The symfony installer requires the php-curl extension or the '
                . 'allow_url_fopen ini setting.');
        }
        return new Client(array('defaults' => $defaults, 'handler' => $handler));

Should i create a pull-request or is the old message "good enough"?

@emil-nasso thanks a lot for working on this issue and for your insightful explanation. I'm closing this issue as "won't fix". Besides, I like your proposal to display a custom error message. Please, consider submitting it as a pull request. Thanks!