参考地址：https://cmsdk.com/php/installing-symfony-project-with-php-7-3-version.html

Tryng to install (and after updating composer):

composer create-project symfony-cmf/sandbox cmf-sandbox

I have those errors:

Installing symfony-cmf/sandbox (2.0.0)

  - Installing symfony-cmf/sandbox (2.0.0): Loading from cache

Created project in cmf-sandbox

Loading composer repositories with package information

Installing dependencies (including require-dev) from lock file

Your requirements could not be resolved to an installable set of packages.

  Problem 1

    - Installation request for phpcr/phpcr-utils 1.3.0 -> satisfiable by phpcr/phpcr-utils[1.3.0].

    - phpcr/phpcr-utils 1.3.0 requires php ^5.6|7.0.x|7.1.x -> your PHP version (7.3.0) does not satisfy that requirement.

  Problem 2

    - phpcr/phpcr-utils 1.3.0 requires php ^5.6|7.0.x|7.1.x -> your PHP version (7.3.0) does not satisfy that requirement.

    - doctrine/phpcr-odm 1.4.2 requires phpcr/phpcr-utils ^1.2.8 -> satisfiable by phpcr/phpcr-utils[1.3.0].

    - Installation request for doctrine/phpcr-odm 1.4.2 -> satisfiable by doctrine/phpcr-odm[1.4.2].

Edit: Continuing the recommendation of Timo, I was able to install the project by executing the following:

composer create-project --ignore-platform-reqs symfony-cmf/sandbox cmf-sandbox

but I have obtained the following errors:

PHP Fatal error:  Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: count(): Parameter must be an array or an object that implements Countable in /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php:101

Stack trace:

#0 /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php(308): Symfony\Cmf\Component\Routing\ChainRouter->all()

#1 /srv/http/symfony/cmf-sandbox/var/cache/dev/appDevDebugProjectContainer.php(1670): Symfony\Cmf\Component\Routing\ChainRouter->setContext(Object(Symfony\Component\Routing\RequestContext))

#2 /srv/http/symfony/cmf-sandbox/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Container.php(335): appDevDebugProjectContainer->getCmfRouting_RouterService()

#3 /srv/http/symfony/cmf-sandbox/var/cache/dev/appDevDebugProjectContainer.php(4955): Symfony\Component\DependencyInjection\Container->get('cmf_routing.rou...')

#4 /srv/http/symfony/cmf-sandbox/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Container.php(33 in /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php on line 101

The project uses a Symfony version 3 structure and may have some incompatibility with PHP 7.3 (as Timo says).

You can tell Composer to ignore the PHP version requirements by adding --ignore-platform-reqs to the composer create-project command.

Else, you'll probably have to wait until the dependencies of symfony-cmf/sandbox get bumped to a later version.