Hello.

I have a Red Hat server with PHP 5.1

How to install JSON?

There isn't php5-json in repositories.

I tried with:

pecl install json
-bash: pecl: command not found

PHP-DEVEL is installed:

yum install php-devel
Loaded plugins: rhnplugin, security
Setting up Install Process
Parsing package install arguments
Package php-devel-5.1.6-23.2.el5_3.i386 already installed and latest version
Nothing to do

yum install pecl
Loaded plugins: rhnplugin, security
Setting up Install Process
Parsing package install arguments
No package pecl available.
Nothing to do

http://www.php.net/json

Check this manual , you can install it easily.
http://forums.crm.saeven.net/showthread.php?t=128

You will have to manually install json as pecl is not available. From php 5.2 json is inbuilt, but you are using 5.1. So you can either manually install it or upgrade to php 5.2 which ever is easiest to you.