Confusing situation happening when using PHP with Mysqli - php

Question

every time I run this code below:
<?php
$servername = "******";
$username = "******";
$password = "*******";
$dbname = "*******";
$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error)
{
die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT date, point, reason, teacher, giver FROM penalty WHERE studentid=".$_GET['id'];
$result = $conn->query($sql);
if ($result["num_rows"] > 0)
{
echo "<table>";
echo "<tr>";
echo "<td>dated</td>";
echo "<td>point</td>";
echo "<td>reason</td>";
echo "<td>giver</td>";
echo "<td>teacher</td>";
echo "</tr>";
while($row = $result->fetch_assoc())
{
echo "<tr>";
echo "<td>".$row["dated"]."</td>";
echo "<td>".$row["point"]."</td>";
echo "<td>".$row["reason"]."</td>";
echo "<td>".$row["giver"]."</td>";
echo "<td>".$row["teacher"]."</td>";
echo "</tr>";
}
echo "</table>";
}
else if ($result["num_rows"] = 0)
{
echo "it is 0!";
}
else
{
echo "Unknown Error!";
}
$conn->close();
?>
I get the error message that I have written like this:
Unknown Error!
So, I'm thinking that the code has some problems in
if ($result["num_rows"] > 0)
What is the problem in the code, and what should I do to solve it?
ps. I'm a newbie in PHP, feel free to answer. Thanks in advance.

Answer 1

To check number of rows return from query we use
$result = $conn->query($sql);
if($result->num_rows > 0)
{
// your code
}
Instead
if ($result["num_rows"] > 0)
Your whole code would be
$result = $conn->query($sql);
if ($result->num_rows > 0) {
echo "<table>";
echo "<tr>";
echo "<td>dated</td>";
echo "<td>point</td>";
echo "<td>reason</td>";
echo "<td>giver</td>";
echo "<td>teacher</td>";
echo "</tr>";
while ($row = $result->fetch_assoc()) {
echo "<tr>";
echo "<td>" . $row["dated"] . "</td>";
echo "<td>" . $row["point"] . "</td>";
echo "<td>" . $row["reason"] . "</td>";
echo "<td>" . $row["giver"] . "</td>";
echo "<td>" . $row["teacher"] . "</td>";
echo "</tr>";
}
echo "</table>";
} else {
echo "it is 0!";
}
Read http://php.net/manual/en/mysqli-result.num-rows.php
Your code is open for sql injection
Read http://php.net/manual/en/mysqli.prepare.php Prepare statement in mysqli