Invalid parameter number: parameter was not defined in [duplicate]

Answer 1

It simple you got a typo:
replace:

$sql = "INSERT INTO `transaction`(`transactionID`, `personalID`, `description`, `cost`, `dateTime`)"
            . " VALUES (:transactinID,:personalID,:description,:cost,CAST(:datetime AS DATETIME)";

with:

$sql = "INSERT INTO `transaction`(`transactionID`, `personalID`, `description`, `cost`, `dateTime`)"
            . " VALUES (:transactionID,:personalID,:description,:cost,CAST(:datetime AS DATETIME)";

Answer 2

you have a typo in your query:

:transactinID should be :transactionID