Hi Alexandre,
This is perhaps a bit late for your needs...
This is based on code that worked for me in 2.7 (changed to reflect your table and column)
In certificate.php add
$myid = $USER->id; // to identify the user in question
$value = $DB->get_record_sql('SELECT * FROM {user_info_data} WHERE id = ?', array($myid)); //query the db
if ($value) {
$value = $value->date; // choose the date column from the $value db answer
} else {
$value = '';
}
$textToDisplay = "This is from the date column: " . $value; // build a text string to be displayed
certificate_print_text($pdf, $x, $y + 258, 'C', 'Helvetica', '', 14, $textToDisplay); //add the custom text. change the positioning and text size to suit
Hope this helps
David