Question

What’s wrong with my statement? I’ve tried everything I can think of.

$stmt = $mysqli->prepare("SELECT host_name, review_title FROM lhr_reviews
              UNION
              SELECT host_url FROM lhr_hostinfo WHERE host_name = ?
              ORDER BY host_name");

$stmt->bind_param("s", $id);
$stmt->execute(); 
$res = $stmt->get_result();

while($row = $res->fetch_assoc()) {
  $list .=  "<li><a href='".$row['host_url']."'>".substr($row['review_title'],0,20)."</a></li>";
}

Answer 1

The columns that are selected from the two queries must match each others, in the number of columns and in the corresponding data types:

The column names from the first SELECT statement are used as the
column names for the results returned. Selected columns listed in
corresponding positions of each SELECT statement should have the same
data type. (For example, the first column selected by the first
statement should have the same type as the first column selected by
the other statements.)

In your query, the first select selects two columns, where the second one selects only one column. This is not true, you can select empty string in the second select as a work around. The other issue is the ORDER BY host_name you can add it this way, you have to put these queries inside a subquery and order in the outer one like this:

SELECT *
FROM
(
    SELECT host_name, review_title FROM lhr_reviews
    UNION
    SELECT host_url, ''            FROM lhr_hostinfo WHERE host_name = ?
) AS sub
ORDER BY host_name;