PHP Stuck on Mysqli_error() - php

Question

I am new to php still learning i have made a navigation with the drop down submenus its working when im inserting new item in navigation but the problem exist when i try to edit the same navigation i get the error in function everything seems fine need help.
![Edit Form][1]
Error:
<code>
Database Query Failed in get subject by idYou have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LIMIT 1' at line 1
</code>
functions.php
<code>
// Get Subject By ID
function get_subject_by_id($subject_id){
global $connection;
$query = "SELECT * FROM subjects WHERE id = {$subject_id} LIMIT 1";
$result_set = mysqli_query($connection, $query);
if(!$result_set){
die("Database Query Failed in get subject by id" . mysqli_error($connection));
}
if($subject_data = mysqli_fetch_array($result_set)) {
return $subject_data;
} else {
return null;
}
}
</code>
edit_subject.php
<code>
<?php include('includes/connection.php'); ?>
<?php require_once('includes/functions.php'); ?>
<?php
if(isset($_POST['submit'])) {
$id = $_GET['subj'];
$menu_name = $_POST['menu_name'];
$position = $_POST['position'];
$visible = $_POST['visible'];
$content = $_POST['content'];
$query = "UPDATE subjects SET menu_name = '{$menu_name}', position = {$position}, visible = {$visible}, content = '{$content}' WHERE id = {$id}";
$result_update = mysqli_query($connection, $query);
if(mysqli_affected_rows($connection) == 1){
$message = "The Subject was successfully created.";
} else {
}
}
?>
<?php
if(isset($_GET['subj'])){
$sel_subject = get_subject_by_id($_GET['subj']);
$sel_page = NULL;
} elseif (isset($_GET['page'])) {
$sel_subject = NULL;
$sel_page = get_page_by_id($_GET['page']);
} else {
$sel_subject = NULL;
$sel_page = NULL;
}
?>
<?php include('includes/header.php'); ?>
<div class="sidebar">
<ul class="sideNav">
<?php
$query_sub = "SELECT * FROM subjects";
$subject_set = mysqli_query($connection, $query_sub);
if(!$subject_set){
die("Database Query Failed1");
}
while($subject = mysqli_fetch_array($subject_set)){
?>
<li><?php echo $subject["menu_name"]; ?>
<?php
$query_page = "SELECT * FROM pages WHERE subject_id = {$subject["id"]}";
$page_set = mysqli_query($connection, $query_page);
if(!$page_set){
die("Database Query Failed2");
} ?>
<ul>
<?php
while($page = mysqli_fetch_array($page_set))
{
?><li><?php echo $page["menu_name"]; ?></li><?php
} ?>
</ul>
</li>
<?php } ?>
</ul>
<br>
+ Add a new Subject
</div><!-- end of sidebar -->
<h2>Edit Subject: <?php echo $sel_subject['menu_name']; ?></h2>
<div class="main">
<br/> <br/>
<?php if(!empty($message)) { ?> <p><?php echo $message; ?></p> <?php } ?>
<form action="edit_subject.php?subj=<?php $sel_subject['id']; ?>" method="post">
<fieldset>
<legend>Edit Subject:</legend>
<p>Subject Name:
<input type="text" name="menu_name" value="<?php echo $sel_subject['menu_name']; ?>">
</p>
<p>Position:
<select name="position">
<?php
$query_opt = "SELECT * FROM subjects ORDER BY position ASC";
$subject_opt = mysqli_query($connection, $query_opt);
if(!$subject_opt){
die("Database Query Failed3");
}
$subject_count = mysqli_num_rows($subject_opt);
for($count=1; $count <= $subject_count+1; $count++){
echo "<option value=\"{$count}\"";
if($sel_subject['position'] == $count){
echo " selected";
}
echo ">{$count}</option>";
}
?>
</select>
</p>
<p>Visible:
<input type="radio" name="visible" value="0"<?php if($sel_subject['visible'] == 0){ echo " checked"; } ?>> No
<input type="radio" name="visible" value="1"<?php if($sel_subject['visible'] == 1){ echo " checked"; } ?>> Yes
</p>
<p>Content:<br/>
<textarea name="content" rows="20" cols="150"><?php echo $sel_subject['content']; ?></textarea>
</p>
<p>
<input type="submit" name="submit" value="Add Subject" class="button-submit">
</p>
</fieldset>
</form>
<br /><br />
</div><!-- end of main -->
<?php include('includes/footer.php'); ?>
</code>

Answer 1

Are you sure that $subject_id in function get_subject_by_id() is not empty ? Try the below code after $query = "SELECT * FROM subjects WHERE id = {$subject_id} LIMIT 1"; in functions.php (only for testing) and make sure that your query is correct.
echo $query;
die();

Answer 2

you've missed the single quotes in the query string.
$query = "SELECT * FROM subjects WHERE id = {$subject_id} LIMIT 1";
should be
$query = "SELECT * FROM subjects WHERE id = '{$subject_id}' LIMIT 1";
youve also done it here.
$query = "UPDATE subjects SET menu_name = '{$menu_name}', position = {$position}, visible = {$visible}, content = '{$content}' WHERE id = {$id}";
should be
$query = "UPDATE subjects SET menu_name = '{$menu_name}', position = '{$position}', visible = '{$visible}', content = '{$content}' WHERE id = '{$id}'";
a note you dont need to wrap in {} for simple variables. You only need them if calling complex variables such as {$array['test']} as expained in this answer