Joomla 3.4.x is not compatible with php 7 as stated in the release
announcement.
The next version 3.5 scheduled for release in Feb will be the first version
that is
On 30 Dec 2015 10:29 pm, "Lyquix" notifications@github.com wrote:

Steps to reproduce the issue

Upgraded to PHP 7.0.1 in Ubuntu 14.04 using the following the steps:
$ add-apt-repository ppa:ondrej/php-7.0
$ apt-get update
$ apt-get install php7.0
$ a2dismod php5
$ service apache2 restart

Load home page of Joomla 3.4.7 site and get HTTP error 500. The following
appears in Apache log:

[Wed Dec 30 16:27:01.296035 2015] [:error] [pid 11346] [client
192.168.56.101:58686] PHP Fatal error: Cannot use Joomla\String\String as
String because 'String' is a special class name in
/srv/www/joomla.lyquix.dev/public_html/libraries/vendor/joomla/registry/src/Format/Json.php
on line 12
Expected result

Home page loaded without errors
Actual result

HTTP error 500

Apache error log shows:

[Wed Dec 30 16:27:01.296035 2015] [:error] [pid 11346] [client
192.168.56.101:58686] PHP Fatal error: Cannot use Joomla\String\String as
String because 'String' is a special class name in
/srv/www/joomla.lyquix.dev/public_html/libraries/vendor/joomla/registry/src/Format/Json.php
on line 12
System information (as much as possible)

Ubuntu 14.04.2 LTS
PHP 7.0.1-4+deb.sury.org~trusty+1 (cli) ( NTS )
Apache server 2.4.7
Joomla 3.4.7
Additional comments

This is the first time I am submitting a bug here. I apologize if I am not
following the right procedure or place. I would like to help.

—
Reply to this email directly or view it on GitHub
#8817.

Got it. Thanks!

I have the exact same issue when I installed gk_portfolio_quickstart_J!3.
The error is :
[pid 16618] [client 127.0.0.1:34022] PHP Fatal error: Cannot use Joomla\String\String as String because 'String' is a special class name in /var/www/html/libraries/vendor/joomla/registry/src/Format/Json.php on line 12, referer: http://take1.unlockyoursoftware.com/administrator/index.php?option=com_admin&view=sysinfo

I am running Joomla 3.7.2. Would the install have messed up something?

I fixed it by extracting the full install over it, then I got back in.

This quickstart is probably not compatible with php 7.

I had the same issue with a Joostrap quickstart recently, I guess a lot of Template developers don't keep their quickstart packages up-to-date, which is fair enough I suppose, as that'd be a fair amount of work.

Dropping the latest Joomla install over the top of the files fixed it fine, as mentioned above.

I fixed the issue by reversing to PHP 5.6 from 7.1, updated Joomla and the plugins and then switched back to PHP 7.1. In my case the Hostgator control panel has a PHP selector to do that. Cheers.