Install Tidy, PHP

Put that line in php.ini next to the other extension=...dll lines and make sure php_tidy.dll is in the directory configured in php.ini under extension_dir.

On Ubuntu, you can add tidy to your php5 installation by running this command in a terminal:

$ sudo apt-get install php5-tidy

If you want to check if it is already installed or validate that the command above worked:

$ php -i | grep libTidy
libTidy Release => 25 March 2009 // means tidy is installed

To install both php5 and tidy at the same time, use this command (as an alternative to the "--with-tidy" configuration option):

$ sudo apt-get install php5 php5-tidy

For PHP 7 just run

sudo apt-get install php-tidy

And restart Apache, That's it :)

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Update: Thanks to @Arnis Juraga

For the version above PHP 7, you can mention the version code as follows.
apt-get install php7.4-tidy

I was having the same trouble, and wrote up my solution here:
Installing Tidy 2.0 PHP extension on Ubuntu 10.10
http://web.onassar.com/blog/2010/11/16/installing-tidy-2-0-php-extension-on-ubuntu-10-10/

It works on newer versions of Ubuntu as well.
While it may not be the most direct route, it works well.
I hope it helps others.