SQL Selection not outputting data - php

Question

Starting off, I'm kind of stumbling in the dark here with PHP/SQL, and don't really know how to bugtest that well, so forgive my vagueness about the exact nature of the problem. Moving on.
I have some code which grabs category ids, names, and descriptions from an SQL table, and saves the result to a variable. This is done through statement preparing to avoid an possibility of SQL injection. This value is then fed into some PHP which checks if the query had any response, and if so prints that into a table.
<?php
//create_cat.php
include_once (__DIR__ . '/../includes/db_connect.php');
include_once (__DIR__ . '/../includes/functions.php');
include_once (__DIR__ . '/header.php');
ini_set('display_errors',1); ini_set('display_startup_errors',1); error_reporting(-1);
$stmt = "SELECT
cat_id,
cat_name,
cat_description
FROM
categories";
if (login_check($mysqli) == true) {
if(!$result = $mysqli->query($stmt)){
echo 'The categories could not be displayed, please try again later.';
} else {
if ($result->num_rows === 0) {
echo 'No categories defined yet.';
} else {
//prepare the table
echo '<table border="1">
<tr>
<th>Category</th>
<th>Last topic</th>
</tr>';
while ($row = $result->fetch_assoc()) {
echo '<tr>';
echo '<td class="leftpart">';
echo '<h3>' . $row['cat_name'] . '</h3>' . $row['cat_description'];
echo '</td>';
echo '<td class="rightpart">';
echo 'Topic subject at 10-10';
echo '</td>';
echo '</tr>';
}
}
}
} else {
echo <<<error
<p>
<span class="error">You are not authorized to access this page.</span> Please login.
</p>
error;
}
include_once (__DIR__ . '/footer.php');
?>
However, the table SQL table definitely has values, but the PHP is only outputting: "The categories could not be displayed, please try again later."

Answer 1

Ok, got it working. I removed $stmt_prep commands, made sure to only use mysqli commands, and fixed some syntax errors. Code still has broken HTML, but the problem I was asking about is fixed.

Answer 2

If you wanted to keep the object oriented style, the main mistake was closing the statement before fetching the results. PHP will free up the statement at the end of the script anyway, but once you've called $stmt->close(); you won't be able to read any data from the query. Since PHP does copy by reference, $result = $stmt; doesn't copy the data, it just references the same closed statement.
The fetch syntax also isn't quite what you had: you need to bind some placeholder variables with eg $stmt->bind_result($name, $description); and then call $stmt->fetch() instead of fetch_assoc.
I'm not clear exactly what login_check does but it would seem to me that you'd want to put this check earlier so that the query is not executed if the user is unauthorized.
Your original code would end up looking something like:
<?php
if (login_check($mysqli) == true) {
$stmt = $mysqli->prepare($prep_stmt);
if ($stmt) {
$stmt->execute();
$stmt->store_result();
if ($stmt->num_rows == 0) {
echo 'No categories defined yet.';
} else {
//prepare the table
echo '<table border="1">
<tr>
<th>Category</th>
<th>Last topic</th>
</tr>';
$stmt->bind_result($name, $description);
while ($stmt->fetch()) {
echo '<tr>';
echo '<td class="leftpart">';
echo '<h3>' . $name . '</h3>'
. $description;
echo '</td>';
echo '<td class="rightpart">';
echo 'Topic subject at 10-10';
echo '</td>';
echo '</tr>';
}
}
} else {
echo 'The categories could not be displayed, please try again later.';
}
} else {
echo <<<error
<p>
<span class="error">You are not authorized to access this page.</span> Please
login.
</p>
error;
}