우분투에서 php5를 설치할 수 없으면 어떻게 해야 합니까?

lhs@taing:/var/www/html$ sudo apt-get install php5
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package php5 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'php5' has no installation candidate
lhs@taing:/var/www/html$ sudo apt-get install libapache2-mod-php5
Reading package lists... Done
Building dependency tree... 50%
Building dependency tree       
Reading state information... Done
Package libapache2-mod-php5 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'libapache2-mod-php5' has no installation candidate