Saving data to database using AJAX and PHP

$(document).ready(function(){
	fetch();

	//ADD
	$('#AddStocks').click(function(){
		$('#AddStocksModal').modal('show');
	});
	$('#addFormstocks').submit(function(e){
		e.preventDefault();
		var addform = $(this).serialize();
		$.ajax({
			method: 'POST',
			url: 'functions/add_stocks_submit.php',
			data: addform,
			dataType: 'json',
			success: function(response){
				$('#AddStocksModal').modal('hide');
				$(this).find('form').trigger('reset');
				if(response.error){
					$('#alert').show();
					$('#alert_message').html(response.message);
				}
				else{
					$('#alert').show();
					$('#alert_message').html(response.message);
					fetch();
				}
			}
		});
	});
	//

<?php

	include_once('../connection/pdo_db_connection.php');

    $assettagad = $_POST['assettagad'];

	$output = array('error' => false);

	$database = new Connection();
	$db = $database->open();

	if(!isset($error)) {
        $stmt = $db->prepare("SELECT asset_tag FROM sys_stocks WHERE asset_tag = ?");
        $stmt->execute(array($assettagad));
        $row_count = $stmt->rowCount();
        
        if ($row_count > 0){
			$output['message'] = 'Asset Tag is already exist!';
        }
	else
    {
        $sql = "INSERT INTO sys_stocks (asset_tag, particulars, status)
             
        VALUES ((UPPER('".$_POST["assettagad"]."')), (UPPER('".$_POST["particularad"]."')), (UPPER('".$_POST["hstatstocks"]."')))";
        ($db->query($sql));
        
        $output['message'] = 'Stock added successfully!';   
    }
}
    //close connection
    $database->close();

    echo json_encode($output);

?>