echo json_encode() is returning NULL - php

Question

I am trying to pass data to a php page via ajax, the data gets inserted to the database, then I need to pick up the last insert and pass the back to update a select menu with that last insert selected. The database gets updated correctly, but Im getting a NULL return for the echo json_echo($data);
Been stuck on this all day, would really appreciate the help!!!
if (empty($_POST) === false && empty($errors) === true) {
$company_id = $_POST['company_id'];
$patient_id = $_POST['addpatient_id'];
$first_name = $_POST['addpatient_firstname'];
$last_name = $_POST['addpatient_lastname'];
$dob = $_POST['addpatient_dob'];
$updated = $_POST['patient_added'];
$update = array();
array_walk($update_data, 'array_sanitize');
foreach($update_data as $field=>$data) {
$update[] = '`' . $field . '` = \'' . $data . '\'';
}
mysql_query("INSERT INTO `lab`.`patients` (`company_id`, `patient_firstname`, `patient_lastname`, `patient_dob`, `patient_added`) VALUES ('$company_id', '$first_name', '$last_name', '$dob', '$updated')");
$last_patient_id = mysql_insert_id();
$result = mysql_query("SELECT `patient_id`, `patient_firstname`, `patient_lastname`, `patient_dob` FROM `patients` WHERE `patient_id` = $last_patient_id");
$data[] = mysql_fetch_assoc($result);
}
echo json_encode( $data );

Answer 1

json_encode returns false if an error happened (php manual). I would start there.
$json_string = json_encode( $data );
if( $json_string ){
echo $json_string;
}else{
echo "Error";
echo "<pre>";
print_r($data);
echo "</pre>";
}
That should at least lead you a way to debug.
EDIT: Also try add this to the beginning of the function all
error_reporting(E_ALL);
ini_set('display_errors', '1');
This will help display errors that the mysql is throwing.
EDIT: I wanted to just fix spelling, but since I need 6 characters minimum I will mention http://jsonlint.com/ to validate what you're putting into json_encode