json - PHP 7.1 json_decode() undefined

i'm try to use composer.But returnFatal error: Uncaught Error: Call to undefined function Composer\Console\json_decode() in /usr/share/php/Composer/Console/Application.php:145 I'm using ubuntu 16.04. and php 7.1.

`apt-get install php7.1-json` -> installed
Copy code

if i execute php -m command no Json in return list . /etc/php/7.1/mods-available/ folder have not anyone .ini file.And i added extension=json.so to php.ini file but did not work

EDIT

etc/php tree

├── 7.0
│   └── apache2
│       └── conf.d
└── 7.1
    ├── apache2
    │   └── php.ini
    └── mods-available
Copy code