Eclipse - PHP Fatal error: Call to undefined function json_decode()

Add PHP 

1. Window->Preferences->Installed PHPs-> Add

    /usr/bin/<search php> select /usr/bin/php7.4.x

2. Check "Window->Preferences->Installed PHPs" checkbox "Use system default php.ini configuration" must be enabled. And I had to setup "apt install php-json" on my Ubuntu.

3. Enable Debug

   Open Terminal -> sudo apt-get install php-xdebug

4. Check "Window->Preferences->Installed PHPs" ->Debuger -> XDebug [from dropdown]

Enjoy