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Because t() is designed for handling code-based strings, in almost all cases, the actual string and not a variable must be passed through t().

Extraction of translations is done based on the strings contained in t() calls. If a variable is passed through t(), the content of the variable cannot be extracted from the file for translation.

Incorrect:

<?php

$message = 'An error occurred.';
drupal_set_message(t($message), 'error');
$output .= t($message);

?>

Correct:

<?php

$message = t('An error occurred.');
drupal_set_message($message, 'error');
$output .= $message;

?>

see http://api.drupal.org/api/function/t/6