$invoice_id = 1;
$safeAddress = "1MJbD7XG6BVvr6TH2XvRJXddB4Q7AAuRCF";
$st = $db->prepare("UPDATE `invoices` SET `btc_address`=? WHERE `invoice_id`=?");
$st->bindParam(1,$safeAddress,PDO::PARAM_STR);
$st->bindParam(2,$invoice_id,PDO::PARAM_INT);

try {
    $st->execute();
}
catch (PDOException $e) {
    echo $e->getMessage();
    exit();
}

UPDATE `invoices` SET `btc_address`=1276VaGEArvc3omNrWMcqaZ7UPcSCADQrt WHERE `invoice_id`=31;

Unknown column '1276VaGEArvc3omNrWMcqaZ7UPcSCADQrt' in 'field list'

UPDATE `invoices` SET `btc_address`= "1276VaGEArvc3omNrWMcqaZ7UPcSCADQrt" WHERE `invoice_id`=31;

$st = $db->prepare("UPDATE `invoices` SET `btc_address`="?" WHERE `invoice_id`=?");

$st = $db->prepare("UPDATE `invoices` SET `btc_address`='?' WHERE `invoice_id`=?");