$list = array(
        'apple',
        'awesome',
        'awards',
        'bike',
        'bbpress',
        'nacin',
);

$current_letter = '';

foreach ( $list as $list_item ) {

        $first_letter = mb_substr( $list_item, 0, 1 );

        if ( $first_letter !== $current_letter ) {

                echo 'Current letter is: ' . $first_letter;

                $current_letter = $first_letter;

        }

        echo $list_item;

}