我正在尝试从PHP 5.5.38升级到PHP 5.6。我遵循SO上的说明，但我收到以下错误信息：
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'
有人知道是什么原因导致了这个问题吗？
P.S.：我使用的是Debian GNU/Linux 7（wheezy）操作系统。