SUGGESTION #1

You could use the MONTH() function

mysql> select MONTH(NOW());
+--------------+
| MONTH(NOW()) |
+--------------+
|            3 |
+--------------+
1 row in set (0.03 sec)

mysql> select MONTH('2013-03-17 02:53:47');
+------------------------------+
| MONTH('2013-03-17 02:53:47') |
+------------------------------+
|                            3 |
+------------------------------+
1 row in set (0.04 sec)

mysql>

You would have to put the MONTH function into the SQL command you are running.

SUGGESTION #2

You could also try the PHP Date Function

$month = date("m",$date)

Also DATE_FORMAT(date_column, '%c') to get '3' as result and DATE_FORMAT(date_column, '%m') to get '03' as result.

First, the condition WHERE date_field >= (CURDATE()-INTERVAL 1 MONTH) will not restrict your results to the current month. It will fetch all dates from 30-31 days ago up to the current date (and to the future, if there are rows with future dates in the table).

It should be:

WHERE date_field >= LAST_DAY(CURRENT_DATE) + INTERVAL 1 DAY - INTERVAL 1 MONTH
  AND date_field < LAST_DAY(CURRENT_DATE) + INTERVAL 1 DAY

Now, to the main question, to create 28-31 dates, even if the table has not rows for all the dates, you could use a Calendar table (with all dates, say for years 1900 to 2200) or create them on the fly, with something like this (the days table can be either a temporary table or you can even make it a derived table, with a somewhat more complicated query than this one):

CREATE TABLE days
( d INT NOT NULL PRIMARY KEY ) ;

INSERT INTO days
VALUES (0), (1), (2), ....
                  ..., (28), (29), (30) ;

SELECT 
    cal.my_date        AS date_field, 
    COALESCE(t.val, 0) AS val
FROM 
    ( SELECT 
          s.start_date + INTERVAL (days.d) DAY  AS my_date
      FROM 
          ( SELECT LAST_DAY(CURRENT_DATE) + INTERVAL 1 DAY - INTERVAL 1 MONTH
                       AS start_date,
                   LAST_DAY(CURRENT_DATE) 
                       AS end_date
          ) AS s
          JOIN days  
              ON  days.d <= DATEDIFF(s.end_date, s.start_date)
    ) AS cal
    LEFT JOIN my_table AS t 
        ON  t.date_field >= cal.my_date 
        AND t.date_field  < cal.my_date + INTERVAL 1 DAY ; 

The above should work for any type of the date_field column (date, datetime, timestamp). If the date_field column is of type DATE, the last join can be simplified to:

    LEFT JOIN my_table AS t 
        ON  t.date_field = cal.my_date ;