mysqli_num_rows() expects parameter 1 to be mysqli_result, null given - php

Question

receiving this error:
mysqli_num_rows() expects parameter 1 to be mysqli_result, null given
Not sure why and have browsed a lot of topics on here trying to figure it out. I know it has something to do with my query, but I just can't put the pieces together.
Thanks for the assistance, I appreciate it!
<?php
session_start();
if (!isset($_SESSION['email'])){
echo "Please login first.";
exit();
}else{
include ("header.php");
require_once ('mysqli_connect.php');
$id=$_GET['id'];
$query = "SELECT * FROM post WHERE post_id=$id";
$result = #mysqli_query ($query);
$num = mysqli_num_rows($result);
if ($num > 0) {
while ($row = mysqli_fetch_array($result, MYSQL_ASSOC)){
?>
<form action="update.php" method="post">
<p>Title: <input name="title" size=50 value="<? echo $row['title']; ?>"></p>
<p>Author: <input name="author" size=50 value="<? echo $row['author']; ?>"></p>
<p>Post: <input name="post" size=50 value="<? echo $row['post']; ?>"></p>
<p>
<input type=submit value=update>
<input type=reset value=reset>
<input type=hidden name="post_id" value="<? echo $row['post_id']; ? >">
</form>
<?
}
}
}
?>

Answer 1

In the line $result = #mysqli_query ($query); the # will silence any errors. Check that $result is not false, null, or some other error before proceeding.
You may want to go without the # until you have it working so you can get more meaningful error messages.

Answer 2

The first thing is that in the last section the code is not properly intented..like value="<? echo $row['post_id']; ?..Then secondly as Joe said the # symbol supress errors..So you need to remove those #symbol from your code..So the code must be ..
<?php
session_start();
if (!isset($_SESSION['email'])){
echo "Please login first.";
exit();
}else{
include ("header.php");
require_once ('mysqli_connect.php');
$id=$_GET['id'];
$query = "SELECT * FROM post WHERE post_id=$id";
$result = mysqli_query ($query);
$num = mysqli_num_rows($result);
if ($num > 0) {
while ($row = mysqli_fetch_array($result, MYSQL_ASSOC)){
?>
<form action="update.php" method="post">
<p>Title: <input name="title" size=50 value="<? echo $row['title']; ?>"></p>
<p>Author: <input name="author" size=50 value="<? echo $row['author']; ?>"></p>
<p>Post: <input name="post" size=50 value="<? echo $row['post']; ?>"></p>
<p>
<input type=submit value=update>
<input type=reset value=reset>
<input type=hidden name="post_id" value="<? echo $row['post_id']; ?>">
</form>
Hope this one helps you ..If any problems regarding then comment below ..:)

Answer 3

I think you must check the value of GET['id'] is having value or not and query is formed with appropriate data.
You might be getting the error because the result is null.
and also check the connection.