Unknown column $cat_name in field list - php

Question

I got the error: unknown column in field list, while I know for sure I haven't made any typos and the columns exist.
Anyone know what I'm overlooking?
<?php
//create_cat.php
include '../includes/connection.php';
$cat_name = mysql_real_escape_string($_POST['cat_name']);
$cat_description = mysql_real_escape_string($_POST['cat_description']);
if($_SERVER['REQUEST_METHOD'] != 'POST')
{
//the form hasn't been posted yet, display it
echo "<form method='post' action=''>
Category name: <input type='text' name='cat_name' id='cat_name'/>
Category description: <textarea name='cat_description' id='cat_description' /></textarea>
<input type='submit' value='Add category' />
</form>";
}
else
{
//the form has been posted, so save it
$sql = 'INSERT INTO categories(cat_name, cat_description) VALUES ($cat_name, $cat_description)';
$result = mysql_query($sql);
if(!$result)
{
//something went wrong, display the error
echo 'Error' . mysql_error();
}
else
{
echo 'New category successfully added.';
}
}
?>

Answer 1

Try this:
$sql = "INSERT INTO categories(cat_name, cat_description) VALUES ('$cat_name', '$cat_description')";
Update:
You used ' to start a string. When doing this its not possible to use variables in the text, they will just be leaved as plain text. But when using " the variables will be evaluated.

Answer 2

The problem is your SQL query. You used single quotation marks, which is for a literal string. Your variables will not be parsed in. You need to use double quotation marks. Not only that, but for strings, you need to put single quotation marks around them when putting them into queries.
$sql = "INSERT INTO categories(cat_name, cat_description) VALUES ('$cat_name', '$cat_description')";
You should also try not to use mysql_* anymore. It's been depreciated (meaning it will be removed from PHP soon). Try looking at MySQLi (very similar to MySQL) or PDO instead.