Can't install php5 on Ubuntu 16.04

TL;DR: If you want to stay with php5, you will need to stay at Ubuntu 14.04 LTS

Using PHP 7.0 on Ubuntu 16.04 LTS

Ubuntu 16.04 has switched to PHP 7.0 with a new infrastructure for PHP package. So, no, you can't install php5 on Ubuntu 16.04, but you can install PHP 7.0 packages with:

apt-get install libapache2-mod-php

That will install a virtual package that depends on the latest PHP version and pull libapache2-mod-php7.0 as a dependency.

If you are looking for extensions, always use a version-less variant as well (e.g. php-apcu instead of php7.0-apcu) as the PECL extensions are packaged without the version to allow smooth upgrades.

Using PHP PPA to coinstall PHP 5.6 with PHP 7.0

There's an option to co-install PHP 5.6 packages using ppa:ondrej/php.

For more information, please see that answer: https://askubuntu.com/a/762161/309221

Extracted from link above:

Assuming libapache2-mod-php is a suitable way to enable PHP in Apache for you, you can proceed in this way:

sudo add-apt-repository ppa:ondrej/php

sudo apt-get update

sudo apt-get install php7.0 php5.6 php5.6-mysql php-gettext php5.6-mbstring php-xdebug libapache2-mod-php5.6 libapache2-mod-php7.0

Switch PHP version ( Apache ):

• from php5.6 to php7.0:
  sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart

• from php7.0 to php5.6:
  sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart

EDIT: the accepted answer does not explicitly say how to co-install PHP 5.6 and 7.0, so I wanted to extend it.

For more information, please see that answer: https://askubuntu.com/a/762161/309221

Extracted from link above:

Assuming libapache2-mod-php is a suitable way to enable PHP in Apache for you, you can proceed in this way:

sudo add-apt-repository ppa:ondrej/php

sudo apt-get update

sudo apt-get install php7.0 php5.6 php5.6-mysql php-gettext php5.6-mbstring php-xdebug libapache2-mod-php5.6 libapache2-mod-php7.0

Switch PHP version ( Apache ):

• from php5.6 to php7.0:
  sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart

• from php7.0 to php5.6:
  sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart