Symfony server not starting

I set up Symfony 3, and when I try to start the server here's what happens:

[heigold1@centos aqua_note]$ php bin/console server:start
[OK] Server listening on http://127.0.0.1:8000    

[heigold1@centos aqua_note]$ (it goes back to the command prompt???) 

[Symfony\Component\Debug\Exception\UndefinedFunctionException]                              
Attempted to call function "posix_setsid" from namespace "Symfony\Bundle\WebServerBundle".  
    server:start [-d|--docroot DOCROOT] [-r|--router ROUTER] [--pidfile PIDFILE] [-h|--help] [-q|--quiet] [-v|vv|vvv|--verbose] [-V|--version] [--ansi] [--no-ansi] [-n|--no-interaction] [-e|--env ENV] [--no-debug] [--] <command> [<addressport>]

And then when I go to http://127.0.0.1:8000 I get the following message:

        This site can’t be reached

        127.0.0.1 refused to connect.

Any help is greatly appreciated.