PDO 'too many connections' when I select all rows from DB - php

Question

I am trying to write a function that verifies a bank transaction amount against all purchases linked to that transaction. It is possible to link a purchase to more than one transaction and for more than one purchase to link to the same transaction.
The function works by totalling the transactions in question, then running a DB query to select all purchases, then checking each purchase to see if it references any of the transactions in question. It then totals the amount of all purchases linked to any of the transactions and reconciles it by checking if the two totals add up.
My problem is that I always get a 'too many connections' error when I get all the purchases from the database. It's fine if I limit the search, but I need all of them in order for the function to work.
I either need to fix the connection overload issue somehow write the query to only search for purchases containing those transactions - but I'm not sure how to do that.
The transactions are comma separated in a column of the purchase entry.
Interestingly, if the list of purchases is called via AJAX it works fine. But if the page (including other prior connections) is loaded statically - it doesn't work. I'm assuming that because the AJAX is loading one thing, there are no prior connections in that instance.
It is quite a complicated system so this may not be very helpful on its own:
public static function verify($ids, $return = NULL) {
// Transaction total
$transactions = explode(",",$ids);
$transTotal = 0;
foreach($transactions as $transaction) {
$transTotal = $transTotal + self::get($transaction,"amount");
}
// Expense/item total
$accounts = AccItem::getAll("all");
$itemTotal = 0;
foreach($accounts as $item) {
$translink = explode(",",$item->transaction_ids);
if(array_intersect($transactions, $translink)) {
$itemTotal = $itemTotal + AccItem::calculate($item->id,"total") + AccItem::calculate($item->id,"tax");
}
}
unset($accounts);
if($transTotal == $itemTotal or $transTotal + $itemTotal == 0) {
if($return) return 'check';
if(!$return) echo '<abbr title="Transaction verified."><i class="fa fa-check-circle"></i></abbr>';
} else {
if(!$return) echo '<abbr title="Transaction amount mismatch!"><i class="fa fa-exclamation-circle"></i></abbr>';
}
}
And here is the getAll function:
public static function getAll($chart_id, $date_from = 0, $date_to = 9999999999999) {
$db = new Data;
if($chart_id == "all") {
$sql = $db->query("SELECT * FROM mc_account_items WHERE date_incurred >= :date_from AND date_incurred <= :date_to ORDER BY date_incurred DESC");
} else {
$sql = $db->query("SELECT * FROM mc_account_items WHERE chart_id = :chart_id AND date_incurred >= :date_from AND date_incurred <= :date_to ORDER BY date_incurred DESC");
$sql->bindParam(":chart_id", $chart_id);
}
$sql->bindParam(":date_from", $date_from);
$sql->bindParam(":date_to", $date_to);
$sql->setFetchMode(PDO::FETCH_CLASS | PDO::FETCH_PROPS_LATE,'AccItem');
$sql->execute();
return $sql->fetchAll();
//unset($db);
}

Answer 1

So I read into things a bit more, turns out I wanted a persistent connection as it was the same connection every time. Instead of opening a new one it will just use the cached connection.
PDO::ATTR_PERSISTENT => true