How to use t() in drupal?

<? php
  $DO_NOT_DO_THIS .= t('This information is ') .'<strong>'. t(' very important') .'</strong>.';?>

This achieves HTML and text separation but makes life hard for you and translators alike. In these simple cases, just include the inline markup, and let the translators live with the fact. It provides them more context instead of the two unique strings you'd have with the above example. Do this instead:

<? php
  $output .= t('This information is <strong>very important</strong>.');?>

Links are the same problem, but their href also brings a few questions, so it is best to take a bit closer look at them. Let's discuss two different types of links, internal and external. Examples of their use are:

<? php
  // DO NOT DO THESE THINGS
  $BAD_EXTERNAL_LINK = t('Look at Drupal documentation at !handbook.', array('!handbook' => '<a href="http://drupal.org/handbooks">'. t('the Drupal Handbooks') .'</a>'));
  $ANOTHER_BAD_EXTERNAL_LINK = t('Look at Drupal documentation at <a href="http://drupal.org/handbooks">the Drupal Handbooks</a>.');
  $BAD_INTERNAL_LINK = t('To get an overview of your administration options, go to !administer in the main menu.', array('!administer' => l(t('the Administer screen'), 'admin'));

  // Do this instead.
  $external_link = t('Look at Drupal documentation at <a href="@drupal-handbook">the Drupal Handbooks</a>.', array('@drupal-handbook' => 'http://drupal.org/handbooks'));
  $internal_link = t('To get an overview of your administration options, go to <a href="@administer-page">the Administer screen</a> in the main menu.', array('@administer-page' => url('admin')));?>