I want to know if BigInt is enough in size. I have created a registration.php where the user gets emailed an account activation link to click to verify his email so his account gets activated.

Account Activation Link is in this format:
PHP Code:
Account Activation Code is in this format:

$account_activation_code = sha1( (string) mt_rand(5, 30)); //Type Casted the INT to STRING on the 1st parameter of sha1 as it needs to be a STRING.

Now, the following link got emailed: http://www.myssite.com/folder/activa...5098477c95c23d

Note the account activation code that got generated by sha1: 22d200f8670dbdb3e253a90eee5098477c95c23d

But in my mysql db, in the "account_activation_code" column, I only see: "22". The rest of the activation code is missing. Why is that ? The column is set to BigInt. Is not that enough to house the Sha1 generated code ? What is your suggestion ?
I changed mysql column type to VARCHAR(40) and then VARCHAR(160) and even to BINARY(40) but no luck.
The sha1 generates the account activation code to 40 digits in the account activation link that gets emailed to the user but the account_activation_code mysql column does not hold that 40 digit value. Only holds the first 2 or 3 digits. What is wrong ?
Using php 5.

Thank You