haebler at gmail dot com (2011-01-22 09:45:52)

On RHEL5/CentOS5 add the EPEL repository (http://fedoraproject.org/wiki/EPEL).
After that, installation is as simple as:
sudo yum install php-pecl-json

cko at audaxis dot com (2010-10-19 06:11:57)

On gentoo, need the USE flag "json" to compile into php

dmitryzayats at NOSPAM dot yahoo dot com (2009-11-20 09:06:04)

Installation steps on CentOs 5.3 include
# yum install php-devel
# yum install php-pear
# yum install gcc
# pear install pecl/json
# cd /etc/php.d
# echo "extension=json.so" > json.ini
# service httpd restart
After that phpinfo() would output
json support enabled
json version 1.2.1

dave at davidhbrown dot us (2009-01-20 04:24:15)

On one server I use (FreeBSD 6.3; PHP 5.2.6 as module in Apache 2.2.10), PHP was compiled with the '--disable-all' configuration option. Consequently, there exists a secondary configuration file -- /usr/local/php5/etc/extensions.ini -- in which the line
; extension=json.so
must be uncommented (delete the semicolon) and Apache restarted for these functions to be available.