为什么这会导致错误的结果呢？

So further to a previous question, I've updated my code to the following

          $stmt = $conn->prepare("SELECT count(id) FROM activities WHERE userId = ? AND status = ? AND DATEDIFF('$date', next_due) >= 1");
          $stmt->bind_param("ss", $userId, $status);
          $stmt->execute();
          $stmt->store_result();
          $numrows = $stmt->num_rows;
          echo $numrows;

It no longer throws an error, but returns $numrows as 1, however I know the number of rows returned should be 2 (I know it should be 2 as I have 2 identical rows in the table for testing purposes). I've tried SELECT count(*), but that also throw a 1 at me.

COUNT() will always return 1 row. either select the result content or use select id.... and then $stmt->num_rows on the full query

This will return number of rows

$stmt = $conn->prepare("SELECT count(id) FROM activities WHERE userId = ? AND status = ? AND DATEDIFF('$date', next_due) >= 1");
$stmt->bindParam('ss', $userId, $status);
$stmt->execute();
$numrows = $stmt->fetchColumn();

See: http://php.net/manual/en/pdostatement.fetchcolumn.php

Thanks for the suggestions. In the end I went with the following;

    $stmt = $conn->prepare("SELECT * FROM activities WHERE userId = ? AND status = ? AND last_completed != ? AND DATEDIFF(?, next_due) >= 1");
    $stmt->bind_param("ssss", $userId, $status, $null, $date);
    $stmt->execute();
    $result = $stmt->get_result();
      echo $result->num_rows;
          ?>