Check wheather Email is Exist and Registration

Email Existence and Registration.php

<?php 
	/*
	@ filter email
	@ check email
	@return json response
	developer: Ariful Islam
	website: phpdark.com
	email: [email protected]
	github.com/arif98741
	expert in php,laravel,codeigniter,node,angular
	
	*/
	if (isset($_POST['checkmail'])) { //give a post request from application
		/*
		*@param1 = host, param2 = username,param3 = password, param4 = databasename
		*/
		$db =  new mysqli("localhost","root","","database"); //
		$response = array(); //this will give response to the application
		

		if ($db) { //if connection true
			$email  = mysqli_real_escape_string($db,$_POST['email']);

			if (filter_var($email, FILTER_VALIDATE_EMAIL)) { //execute if email is valid
			  	$query = "select * from registration where email='$email'";
				$stmt = $db->query($query) or die($db->error)."error at line number ".__LINE__;

				if ($stmt) {
					if ($stmt->num_rows > 0) { //check wheather email is exist
						$response['error'] = "Email is already exist";

					} else{

						$insertQuery = "select into registration(name,email,address) values('jhon','[email protected]','new york city')";
						$insertStmt = $db->query($insertQuery) or die($db->error)."error at line number ".__LINE__;
 
						if ($insertStmt) { //if data insert successful

							$response['message'] = "Successfully inserted to database";
						} else { //if any error occurs

							$response['error'] = "Failed to insert data";
						}
					}
				} else { //failed query has error
					$response['error']  = "Failed to execute query";
				}
			} else { //assign value to error index of response array
			  $response['error'] = "Email is not valid";
			}


		} else {
			$response['error'] = "Failed to connect to database";
		}

		echo json_encode($response);
	} 


?>