MySQLi SUM function with pound/dollar sign?

Within my Admin panel, I would like to see all profits and sales by Weekly, Monthly, Yearly and Total.

So far I am able to display the transactions recorded in the ap_payment_logs database in to an advanced responsive table. Above this is where I am trying to display the profits for each margin: Weekly, Monthly Yearly and Total. By selecting and calculating the column: txn_amount.

This all works with normal numbers added to that column, although the way my site works, it adds a Pound sign (£) before the digits:

£10.00, £20.00, £50.00...

The SUM function doesn't seem to work with this Pound sign, so I am wondering if there is any simple way to perform the same calculations which would work for this? Here is my current code:

$result = $conn->query('SELECT SUM(txn_amount) AS value_sum FROM `ap_payment_logs` WHERE `txn_date` >= NOW() - INTERVAL 7 DAY'); 
                $row = mysqli_fetch_assoc($result); 
                $sum = $row['value_sum'];
                echo $sum;

Please Note: I cannot remove the addition of the £ sign to achieve my results.

Thanks in advance!

Solution

Having a Table like this:

create table dummy (
   id int primary key auto_increment, 
   test varchar(10)
);

and these rows in the table:

mysql> select * from dummy;
+----+---------+
| id | test    |
+----+---------+
|  1 | 10.0    |
|  2 | 14.0    |
|  3 | €14.0   |
+----+---------+

you could get the sum of the test values with the following select statement:

select sum(replace(test, '€', '')) from dummy;

Anyway like Mark Baker I would recommend to store your numeric colums in numeric fields as this has a couple of advantages (aggregates like sum are easier, sorting works...).