jquery Ajax not saving to mysql - php

Question

I have a php file that has jquery ajax on it to submit a form and save its contents into a mysql without doing a page refresh.
My issue is if I load the file that the ajax call does the contents are saved into the database however if I try to use the form nothing gets saved to the database even though I get a success message.
Here is my code
The HTML file
<div id='backinstock-form'>
<form>
<input id='instockemailadr'>
<input id='instockpid' value='5' type='hidden'>
<input name='submit' class='submitbackinstock' type='button' value='Submit'>
</form>
<span class='error' style='display:none'> Please Enter A Valid Email</span>
<span class='success' style='display:none'> Email Submitted Success</span>
</div>
require(['jquery'], function($)
{
$(".submitbackinstock").click(function()
{
var instockemailadr = $("#instockemailadr").val();
var instockpid = $("#instockpid").val();
var dataString = 'usereaddr='+ instockemailadr + '&sku=' + instockpid;
if(instockemailadr =='' || instockpid =='')
{
$('.success').fadeOut(200).hide();
$('.error').fadeOut(200).show();
}
else
{
//console.log(dataString);
var dataString = new formData();
dataString.append('usereaddr', instockemailadr);
dataString.append('sku',instockpid);
$.ajax({
type: "POST",
url: "<?php echo $block->getBaseUrl(); ?>/pub/media/theme_customization/backin-stock-email-code/backinstock_process_email.php",
data: dataString,
success: function(){
$('.success').fadeIn(200).show();
$('.error').fadeOut(200).hide();
}
});
}
return false;
});
});
The PHP File
<?php
// MySQL portion
$conn = mysqli_connect("localhost", "root", "root");
mysqli_select_db($conn,"multisitedb");
$usereaddr = mysqli_real_escape_string($conn, $_POST['usereaddr']);
$stockproductid = mysqli_real_escape_string($conn, $_POST['sku']);
$created_on = date('Y-m-d h:i:s');
$result = mysqli_query($conn, "INSERT INTO multisitedb.backinstock_email_addresses(email_addr, product_sku, created_on)
VALUES ('$usereaddr','$stockproductid','$created_on')");
if($result) {
echo $result; // or whatever you want
} else {
echo "Something went wrong.";
}
?>

Answer 1

First, you didn't specify a Database in your php script. The format is mysqli_connect(host, username, password, database);
Secondly, your ajax code seems wrong. You're making a post request. The data should be an instance of formData(). You can modify this way:
var dataString = new formData();
dataString.append('usereaddr', instockemailadr);
dataString.append('sku',instockpid);
//...
data: dataString,
success: function(){
Also, your PHP script is not doing security checks. You need to validate user inputs as well as use prepared statements to prevent against SQL injection attack.