Install composer

[betatester3.0]

The easiest way to deal with this should be to just disable suhosin for that domain.
Log in as zadmin, go to Module Admin > Apache Config > Override a Virtual Host > [select domain] > untick 'Suhosin Enabled' and save.

Unless, @[Me.B] or @[Ron-e] has a better idea.

TGates & livecreation I've got a better idea, his using the incorrect composer.json or he did not enable the .json script.
He first needs to enable/whitlist the composer in the php.ini file.

Code:

USE ONLY IF YOU CANNOT ACCESS PHP.INI FILE
php -d option=value_to_what_should_be_added_on_the_fly_in_php.ini

Enable/Whitelist the composer 

add this on the first line in your php.ini file

Code:

suhosin.executor.include.whitelist = phar

locate the composer.json and then attempt to start it, that should work if you have a working composer.json file