PHP Arrays, Serialization and Unserialization - php

Question

I have a problem. I have a website with people and different transactions they make to and from a fake online bank. I want to be able to store an array of each person's transactions on my mysql database. I want each transaction to be defined as an associative array with a timestamp and the sql query that represents their transaction with the "bank".
Then I want those, after being serialized, to be the values of a transactions array that holds all of their transactions. Then I want to serialize that and store it in the database so that later I can add a transaction by unserializing it and appending a serialized array of another transaction to it. So far this code below works except that it just overwrites the one transaction and doesn't append a new one. I'd really appreciate any help.
Thanks in advance
function modify_transactions($row, $sql)
{
$sql=mysql_real_escape_string($sql);
if(isset($row["TRANSACTIONS"]))
{
$transactions = unserialize($row["TRANSACTIONS"]);
}
else
{
$transactions = array();
}
$transaction_array = array("timestamp"=>time(),"query"=>$sql);
$transaction_data = serialize($transaction_array);
$transactions[] = $transaction_data;
$transactions_upload = serialize($transactions);
$name = $row["NAME"];
$query = "UPDATE band.students SET TRANSACTIONS = '$transactions_upload' WHERE students.NAME = '$name'";
mysql_query($query);
}

Answer 1

If I were you, I'd rather go for a new table where every entry would represent a transaction and that had a foreign key student_id.
That'd be much, much, much cleaner and more flexible and scalable (i.e. what if you want to show the last 3 transactions of user X? What if a user had several million transactions?).

Answer 2

First, you don't need to serialize each array, then serialize again. Serialize is recursive:
$array = array(
array(
'1',
array()
),
array(
'2',
array()
)
);
$serialized = serialize($array);
$unserialized = unserialize($serialized);
echo "<pre>";
print_r($unserialized);
echo "</pre>";
Prints:
Array
(
[0] => Array
(
[0] => 1
[1] => Array
(
)
)
[1] => Array
(
[0] => 2
[1] => Array
(
)
)
)
So just serialize right before inserting into the database.
Second, you should change your database structure. Like vzwick mentioned, create a new table with a foreign key of the student. That way each entry represents a transaction.
Also, why are you storing the actual SQL query? That doesn't make any sense to me. Why don't you actually make a fake transaction?