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While I’ve added custom fields to a user’s registration on a site of mine, I want to use some custom code to login to a SQL database and check the given ID with the IDs in the database. I’ve already got code to do so,

<?php
include_once('dbconnect.php');
//if(isset($_POST['search'])){
	$q = $_POST['q'];
	$q = $_GET['query']; 
	session_start();
		session_start();	
		session_start();

	$query = mysqli_query($conn,"SELECT * FROM <code>Persons</code> WHERE <code>id</code> LIKE '%$q%'"); 
//Replace table_name with your table name and <code>thing_to_search</code> with the column you want to search
	$count = mysqli_num_rows($query);
	if($count != "1"){
		$output = '<h2>No result found!</h2>';
	}else{
		while($row = mysqli_fetch_array($query)){
		$s = $row['name']; // Replace column_to_display with the column you want the results from
				$output .= '<h2> Found: '.$s.'</h2><br>';
			}
		}
		$_SESSION['name'] = $s;
		$_SESSION['id'] = $q;
		$_SESSION['fromMain'] = "true";
		echo $_SESSION['fromMain'];
	//}
?>

How exactly would I connect the above PHP code to the registration of a potential user? Additionally, would there need to be any changes to the code to make it work?

WP: 5.1.1
BP: 4.2.0
Site: http://worldcubers.com/