MYSQLi query error in a form - php

Question

I can't seem to find the error. This form is supposed to update a category in the database.
When I try to update the category it gives me this error
Notice: Undefined variable: cat_id in C:\xampp\htdocs\CMS_project\admin\categories.php on line 90
QUERY FAILED!You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
This is the code below, i've commented where is the line 90.
<form action="categories.php" method="post">
<div class="form-group">
<label for="cat_title">Update Category</label>
<?php
if(isset($_GET['edit']))
{
$cat_id = $_GET['edit'];
$query = "SELECT * FROM categories WHERE cat_id = {$cat_id} ";
$select_categories_id = mysqli_query($connection, $query);
while($row = mysqli_fetch_assoc($select_categories_id))
{
$cat_id = $row['cat_id'];
$cat_title = $row['cat_title'];
?>
<input value="<?php if(isset($cat_title)){echo $cat_title;} ?>" type="text" name="cat_title" class="form-control">
<?php } } ?>
<?php
if(isset($_POST['update_category']))
{
$the_cat_title = $_POST['cat_title'];
//Below is line 90
$query = "UPDATE categories SET cat_title = '{$the_cat_title}' WHERE cat_id = {$cat_id} ";
$update_query = mysqli_query($connection, $query);
if(!$update_query)
{
die("QUERY FAILED!" . mysqli_error($connection));
}
}
?>
</div>
<div class="form-group">
<input type="submit" name="update_category" value="Update Category" class="btn btn-primary">
</div>
</form>
This is where the form gets the ID
<?php
while($row = mysqli_fetch_assoc($select_categories))
{
$cat_id = $row['cat_id'];
$cat_title = $row['cat_title'];
echo "<tr>
<td>{$cat_id}</td>
<td>{$cat_title}</td>
<td><a href='categories.php?delete={$cat_id}'>Delete</a></td>
<td><a href='categories.php?edit={$cat_id}'>Edit</a></td>
</tr>";
}
?>

Answer 1

The $cat_id variable is simply not defined in the scope of your query. This means, that the $cat_id variable is only declared inside if statement where you check if $_GET['edit'] is set.
To fix it, add a hidden field with category id to your form, and then in PHP add:
$cat_id = $_POST['cat_id'];
just before the 90th line.
By the way remember to use mysqli_real_escape_string to prevent SQL-incjection issues.

Answer 2

I figured it out.
I had to delete the categories.php from the action in form, if I leave it blank it works fine. :)