PHP Shows error when data inputted exist - php

Question

I am currently learning PHP Programming,
I wanted to show error when the inputted data exists.
Can anyone show some example on how it doest work?
Here is code I used:
<?php
ob_start();
session_start();
require '../database/database.php';
// require '../database/inc_config2.php';
require ('p_nav.php');
if( !isset($_SESSION['user']) ) {
header("Location: index.php");
exit;
}
// select loggedin users detail
$res=mysql_query("SELECT * FROM users WHERE userId=".$_SESSION['user']);
$userRow=mysql_fetch_array($res);
//Variable input data
//include_once 'dbconfig.php';
if(isset($_POST['btn-save']))
{
// variables for input data
$pcat = $_POST['pcat'];
if($pcat=='')
{
//javascript use for input checking
echo"<script>alert('Please enter Category')</script>";
// echo 'Empty Category';
header ("location:..\product\p_cat.php");
exit();
}
// variables for input data
//check if category is exist
//here query check weather if user already registered so can't register again.
$check_pcat_query="select * from pcategory WHERE pcat='$pcat'";
// $check_user_query="select * from users WHERE user_name='$user_name'";
$run_query=mysqli_query($dbcon,$check_pcat_query);
if(mysqli_num_rows($run_query)>0)
{
//echo "<script>alert('Category $pcat is already exist in our database, Please try another one!')</script>";
exit();
}
// sql query for inserting data into database
//temporary disabled
$sql_query = "INSERT INTO pcategory(pcat) VALUES('$pcat')";
mysql_query($sql_query);
//echo "Supplier Added Successfully";
//header ("location:..\product\index.php");
exit();
/*
$insert_user="insert into pcategory (pcat) VALUE ('$pcat')";
if(mysqli_query($dbcon,$insert_user))
{
// echo"<script>window.open('welcome.php','_self')</script>";
echo "Welcome";
} */
}
// sql query for inserting data into database
?>

Answer 1

There is an error, you are using both MySQL and MySQLi, using both is fatal error if you work on live server. Just focus on MySQLi as MySQL is deprecated officially and is not supported anymore.
Now come to your question. Your logic seems to be fine a bit. This is how you have to implement:
Fetch values from FORM
First clean the input using mysqli_real_escape_string() function.
Use above values and check the record existance in the database (here use SELECT statement)
If mysqli_num_rows() gives you result greater than 0 than;
(a) it means record already exists
(b) else, execute INSERT statement