Reversing AES_ENCRYPT Difficulties - php

Question

I am consistently receiving null whenever I try to reverse the cipher text that I store in a MySQL database through PHP.
The PHP code used to insert the data:
public function insertChildren($employeeID, $empData, $key, $childName, $childBirth, $childGender, $childSSN, $isStep, $isFoster, $isStudent, $isHandicap, $address) {
$conn = $this->connect('insurance');
$insertChildren = $conn->prepare('INSERT INTO dependent_children (emp_id, ssn, name, dob, gender, handicap, student, foster, step, address) VALUES (:emp_id, AES_ENCRYPT(:ssn, AES_ENCRYPT(:key, UNHEX(sha1(:empData)))), :name, :dob, :gender, :handicap, :student, :foster, :step, :address)');
$insertChildren->bindParam(":emp_id", $employeeID, PDO::PARAM_INT);
$insertChildren->bindParam(":name", $childName, PDO::PARAM_STR);
$insertChildren->bindParam(':dob', $childBirth, PDO::PARAM_STR);
$insertChildren->bindParam(':empData', $empData, PDO::PARAM_STR);
$insertChildren->bindParam(':gender', $childGender, PDO::PARAM_STR);
$insertChildren->bindParam(':key', $key);
$insertChildren->bindParam(':ssn', $childSSN, PDO::PARAM_LOB);
$insertChildren->bindParam(':handicap', $isHandicap, PDO::PARAM_STR);
$insertChildren->bindParam(':student', $isStudent, PDO::PARAM_STR);
$insertChildren->bindParam(':foster', $isFoster, PDO::PARAM_STR);
$insertChildren->bindParam(':step', $isStep, PDO::PARAM_STR);
$insertChildren->bindParam(':address', $address, PDO::PARAM_STR);
$insertChildren->execute();
}
The SQL query that I thought would reverse it:
SELECT CAST(AES_DECRYPT(ssn, AES_DECRYPT('/ same random hexadecimal key bound in the php statement / ', unhex(sha1('1234')))) AS CHAR(50)) from dependent_children
Please note 1234 is the value that should be bound to the empData field in the PHP. I thought the latter query would correctly decrypt the first, but such is the case. Instead, I receive null. I am sure it is something simple, but I have not been able to locate the error source. Thanks so much!