无法通过正则表达式找到任何包 'php5.6'

Couldn't find any package by regex 'php5.6'

我正在尝试从 PHP 5.5.38 升级到 PHP 5.6。我正在按照说明 进行操作，但收到以下错误消息：

sudo apt-get -y install php5.6 php5.6-mcrypt php5.6-mbstring php5.6-curl php5.6-cli php5.6-mysql php5.6-gd php5.6-intl php5.6-xsl
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package php5.6
E: Couldn't find any package by regex 'php5.6'
E: Unable to locate package php5.6-mcrypt
E: Couldn't find any package by regex 'php5.6-mcrypt'
E: Unable to locate package php5.6-mbstring
E: Couldn't find any package by regex 'php5.6-mbstring'
E: Unable to locate package php5.6-curl
E: Couldn't find any package by regex 'php5.6-curl'
E: Unable to locate package php5.6-cli
E: Couldn't find any package by regex 'php5.6-cli'
E: Unable to locate package php5.6-mysql
E: Couldn't find any package by regex 'php5.6-mysql'
E: Unable to locate package php5.6-gd
E: Couldn't find any package by regex 'php5.6-gd'
E: Unable to locate package php5.6-intl
E: Couldn't find any package by regex 'php5.6-intl'
E: Unable to locate package php5.6-xsl
E: Couldn't find any package by regex 'php5.6-xsl'

有谁知道是什么导致了这个问题？

P.S.：我在使用 Debian GNU/Linux 7（气喘吁吁）

我在这里找到了正确的说明：http://nwdthemes.com/2016/01/31/how-to-upgrade-to-php-5-6-on-debian-7-wheezy/