We all know Red Hat that is one of the most popular commercial Linux. But if you can find RHEL (Red Hat Enterprise Linux) ISO, you can use it without any cost except you can't install or update any software from Red Hat Repository because you don't have Red Hat Subscription

We known Red Hat was develop based on Linux and license of Linux is GNU GPL. One rule of GPL force you to open your modified source code to the public. Red Hat did as rule say but they didn't tell how to compile them.

CentOS (Community ENTerprise Operating System) is Linux Distro that got code from Red Hat and find the way to compile it. So the different between CentOS and Red Hat are just only brand and some Red Hat proprietary software. Also the version will be the same too. (RHEL 6 vs. CentOS 6)

This article will focus on installation CentOS 6 64-bit with minimal software as possible. This will suitable with who want to use CentOS as a stable server. You can download minimal ISO from [here](http://mirror1.ku.ac.th/centos/6/isos/x86_64/CentOS-6.0-x86_64-minimal.iso

Lately I have project to research about Zimbra for Email Server. But OS that Zimbra support just Red Hat Enterprise Linux, SUSE Linux Enterprise Server and Ubuntu Server only. If we consider about cost so we decide to use Ubuntu Server

We are many known about Ubuntu Desktop that is the very popular Linux for end user. But also there is version that specific for server too. This how to step-by-step will focus on Ubuntu Server 10.04 LTS (Lucid Lynx) 64-bit that Zimbra supported. You can download ISO from here.

After all the administrator install linux. The next thing that we have to install and setting instantly is Secure Shell Server or we known in short SSH Server

In this article I will explain how to secure SSH Server by refer to Debian distro that I familiar with

• Sure thing that we have to install linux first
• Then install SSH Server with command

apt-get install openssh-server

• edit file /etc/ssh/sshd_config to config SSH Server. I will show only config that related to security

See this code

if($string == 'word1' || $string == 'word2')
{
  // Do Something
} elseif($string == 'word3' || $string == 'word4' || $string == 'word5' || $string == 'word6' || $string == 'word7') {
  // Do Something
}

First if still ok to use but in next elseif is too long! So I tried to find better way to compare many string and in_array() function is the answer.

if(in_array($string, array('word1', 'word2')))
{
  // Do Something
} elseif(in_array($string, array('word3', 'word4', 'word5', 'word6', 'word7'))) {
  // Do Something
}

From this code

if($login)
  echo 'Welcome'.$username;
else
  echo 'Welcome Guest';

We can use short if-else for compact and effective by below code

echo 'Welcome '.($login ? $username : 'Guest');