Your php.ini contains a list of "extension=somefile.so" lines. You are missing the correct line for your tidy extension. It should be included but commented in your file. Just remove the ; character in front of the one line for the tidy extension and restart your webserver.
Sample for windows:
extension=php_tidy.dll
Sample for Linux:
extension=tidy.so
On some Linux distribution, it has to be installed if you can't find the extension file tidy.so
in your extensions folder. Execute the correct line depending on your distribution. You have to be root or use sudo:
CentOS/RedHat (replace with correct lib as found via yum search php-tidy
:
yum install rh-php56-php-tidy.x86_64
Debian/Ubuntu:
apt-get install php5-tidy