[Linux] Apache: Invalid command php_admin_value error perhaps misspelled or defined..

If you get this error:

“Invalid command php_admin_value, perhaps misspelled or defined by a module not included in the server configuration
Action configtest failed.
The Apache error log may have more information.
fail!”

follow the steps.

First, install the missing package.
Run the command:
# sudo apt-get install libapache2-mod-php5

Activate the module in Apache.
Run the command:
# sudo a2enmod php5

Restart the apache server
Run the command:
# sudo /etc/init.d/apache2 restart

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