I have installed Composer and followed the Quick start guide in the Laravel 4 documentation.
I get the following error when I run composer
Having the setting xdebug.scream = 1
in the configuration was the cause of the problem for me. I fixed it by doing the following:
Find XDebug configuration file.
$ sudo find / -name xdebug.ini
Edit file using any text editor.
$ sudo vi /your_path/xdebug.ini
Set xdebug.scream = 0
Reload the server (Apache/Nginx/whatever).
$ sudo service nginx reload