Inside the block admini interface, one can enter PHP code snippets in “Page visibility settings” section of the block configuration page.
While a page being generated, Drupal will run the PHP snippet to determine whether a block will be displayed.

Examples of some of the most common snippets follow; each snippet should return TRUE or FALSE to indicate whether the block should be visible for that particular request.

Displaying a Block to Logged-In Users Only
Only return TRUE when $user->id is not 0.

<?php
   global $user;
   return (bool) $user->uid;
?>

Hooks allow modules to interact with the Drupal core.

Module system of Drupal is based on the concept of "hooks". A hook is actually a PHP
function that is named modulename_hookname(), where "modulename" is the name of the module (whose
filename will be modulename.module) and "hookname" is the name of the hook. Every hook have
a defined set of parameters and a specified result type.

It is always possible to check if the user is logged in as admin using the drupal built in user_access() function, super administrator account doesn't have a role by default so you can check it using :

<?php
if(user_access('administer')) 
{....your code goes here....}
?>

However, in case if you want to check whether the logged-in user has a certain role. For example, something like this if you have a role called 'moderator'

<?php
global $user;
if(is_array($user->roles) && in_array('moderator', $user->roles))

{... your code goes here ...}

?>

The correct way of testing if a user is logged in is to test whether $user->uid is 0:

global $user;
if ($user->uid) {
   $output = t('User is logged in!');
else {
   $output = t('User is an anonymous user.');
}

This approach is often used when defining a block that shows one thing to
logged-in users and something else to anonymous users:

<?php
   global $user;
   if ($user->uid) {
     return t('You are currently logged in!');
   }
   else {
     return t('You are not currently logged in.');
   }
?>

Sometimes we need to connect to third-party or legacy databases,and it would be great to use Drupal’s database API for doing this.

In the settings.php file, $db_url can be either a string (as it usually is) or an array composed of multiple database connection strings. Here’s the default syntax, specifying a single connection string:

$db_url = 'mysql://username:password@localhost/databasename';

But while using an array, the key is a shortcut name you will refer to while activating the data-base connection, and the value is the connection string itself.