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$servername = "localhost";
$username = "root";
$password = "";
$dbname = "dbname";

$conn = mysqli_connect($servername, $username, $password, $dbname);

$product_name = $_POST['product_name'];
$category_name = $_POST['category_name'];
$date = $_POST['date'];
$quantity = $_POST['quantity'];
$unit_price = $_POST['unit_price'];
$vendor_name = $_POST['vendor_name'];

$sql = "INSERT INTO purchases (product_name,category_name,date,quantity,unit_price,vendor_name) VALUES 

header("location: ../purchases.php?submit=success");


What I have tried:

When I click the submit button it shows the error above from lines 11 through to line 16 and then 22
Updated 31-Oct-20 9:06am

When I click the submit button it shows the error above from lines 11 through to line 16 and then 22

Unfortunately, we can't do much for you because problem is not in this code.
Problem only become visible in this code.
This code process data from an input form, it expect data in $_POST using spùe indexes named after input fields, problem is that those indexes do not exist.
The only thing to do is either inspect the contain of $_POST with debugger or print its contain with key/values pairs. And check code of input form.
$sql = "INSERT INTO purchases (product_name,category_name,date,quantity,unit_price,vendor_name) VALUES 

Not a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]
SQL Injection Attacks by Example[^]
PHP: SQL Injection - Manual[^]
How can I explain SQL injection without technical jargon? - Information Security Stack Exchange[^]
Check your HTML code: the chance are you have misspelled you indexes, or have no matching controls on your page. This explains it with simple sample code: PHP $_POST[^]

And as Patrice T has said: Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

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