如何在ubuntu 15上安装pcntl扩展

我试图在ubuntu 15上安装pcntl，当我试图获得php5源代码时，我得到了这个

我所遵循的程序是

sudo apt-get source php5
cd php5(release version)/ext/pcntl
sudo phpize
sudo ./configure
sudo make
sudo make install

但在

sudo apt-get source php5

I am getting

Reading package lists... Done
Building dependency tree
Reading state information... Done
NOTICE: 'php5' packaging is maintained in the 'Git' version control system at:
git://anonscm.debian.org/pkg-php/php.git
WARNING: The following packages cannot be authenticated!
  php5
E: Some packages could not be authenticated

服务器是ubuntu Server 15.04

PHP 5.6.4-4ubuntu6.3 (cli) (build: Sep 29 2015 12:44:47)