To install/enable Json support for PHP on a Linux Plesk server, follow the below steps:

1. Install php-pear and gcc

PHP Code:
    # yum install php-pear gcc 
2. Install json module

PHP Code:
    # pecl install json 
3. Now, the json.so file is created under the php lib directory i.e. /usr/lib64/php/modules/ and needs to be called in the json.ini so PHP configuration could read it.

PHP Code:
# cd /etc/php.d 

# echo "extension=json.so" >> json.ini 
4. Once done, restart the httpd service

PHP Code:
# /etc/init.d/httpd restart 
This is it.

However, the other way is to manually install the php-json module. Follow the below steps:

1. Download the json package

PHP Code:
# wget http://pecl.php.net/get/json-1.2.0.tgz 
2. Untar and goto the newly created directory

PHP Code:
# tar -zxf json-1.2.0.tgz 

# cd json-1.2.0 
3. Create the configuration files and configure json

PHP Code:
# phpize 

# ./configure 
4. Create the installation file and install it
PHP Code:
# make 

# make install 
Once done, json.so will be created in /usr/lib64/php/modules/ directory and needs to be called by the php configuration directory
PHP Code:
# cd /etc/php.d 

# echo “extension=json.so” >> json.ini 
Restart the httpd service and check the php-json module by executing

PHP Code:
# php -m | grep json 
Note: The manual installation method can be used on a plain Linux server as well, just the extension need to be added in the php.ini file directly.