PHP Fatal error: Call to undefined function json_decode()

Using Ubuntu?

Short answer:

sudo apt-get install php7.2-json

(or php7.1-json or php5-json depending on the PHP version you're running)

Then of course make sure you restart Apache:

sudo service apache2 restart

Or if you are using PHP-FPM:

sudo service php7.2-fpm restart

(Or php7.1-fpm or php5-fpm)

Explanation

Debian has removed the previous JSON extension as of PHP 5.5rc2 due to a license conflict.

The JSON license has a clause which states:

The Software shall be used for Good, not Evil.

This causes a problem with Free Software Foundation's definition of free software which states:

The freedom to run the program, for any purpose (freedom 0).

FSF goes on to specifically list the JSON license as nonfree.

Yes it seems a bit silly. Nevertheless Debian has removed the non-compliant JSON extension, and instead offered a replacement extension that is functionally equivalent.

To be clear: PHP itself has NOT removed JSON, it's still in master. This is a distro / package manager issue.

Rasmus makes it pretty clear:

We have not removed json and we will never release a version of php without json support built in. Any changes in 5.5 is due to whatever distro packaging you are using which we have no control over.

More details

http://iteration99.com/2013/php-json-licensing-and-php-5-5/

http://liorkaplan.wordpress.com/2013/06/01/bye-bye-non-free-php-json-extension/

https://bugs.php.net/bug.php?id=63520

http://philsturgeon.co.uk/blog/2013/08/fud-cracker-php-55-never-lost-json-support

With Ubuntu :

sudo apt-get install php5-json
sudo service php5-fpm restart