Prepared statement and null value when using? - php

Question

I have a need to evaluate if a user logged in to a system after a specific date. To do this, there are three tables in a MySQL database, users, survey and logins. Survey holds the date of a point in time that needs compared against the users last log in. Here's the question.
When I used the "?" placeholder, the resulting num_rows count was always 0. But when I assign the values before handing the query statement to $mysqli->prepare(), the process works as expected. Somehow, store_result() was not picking up the column. Here is my code:
if (isset($userId)){
//get survey release date
$res3 = $mysqli->query("SELECT sur_date,sur_url FROM survey ORDER BY sur_id DESC limit 1");
$array = $res3->fetch_assoc();
$theta_date = $array['sur_date'];
//$theta_date = "2013-01-18 01:00:00";
//this didn't generate errors, but didn't output the correct result either.
//$query = "SELECT login_id FROM logins WHERE login_user=? AND login_date>=?";
//if ($stmt = $mysqli->prepare($query)){
// $stmt->bind_param('ss',$userID,$theda_date);
// $stmt->execute();
//this works
$query = "SELECT login_id FROM logins WHERE login_user='$userId' AND login_date>='$theta_date'";
if ($stmt = $mysqli->prepare($query)){
$stmt->execute() or die("The query did not work");
//if number is greater than 0 do something
$stmt->store_result();
printf("The number of login ids after theta are %d",$stmt->num_rows);
$stmt->close();
}else{
echo "The query did not execute.";
}
}else{
echo "The User ID was not valid.";
exit();
}
$mysqli->close();
Any insight would be helpful,

Answer 1

The prepared statement seems to be having an issue with the $theta_date datetime format. $theta_date is stored in the survey table as '2013-01-18 01:00:00'. bind_param() was trying to parse $theta_date as a reference. Here is the solution:
//convert datetime to Unix timestamp with strtotime()
$theta_date = $array['sur_date'];
$timestamp = strtotime($theta_date);
//In the prepared statement, use MySQL FROM_UNIXTIME function to convert timestamp
$query = "SELECT login_id FROM logins WHERE login_user=? AND login_date>=FROM_UNIXTIME(?)";
//changed the parameter types to integer and bound $timestamp to second placeholder
if ($stmt = $mysqli->prepare($query)){
$stmt->bind_param('ii',$userId,$timestamp);
$stmt->execute();
//the rest is the same
$stmt->store_result();
printf("The number of login ids after theta are %d",$stmt->num_rows);
$stmt->close();
}
That was a pain.