PDO error: no database selected - php

Question

I'm making a register script and using PDO for the first time, but I get this error when trying to use it. The defines works fine when use MySQLi.
My code gives me this error:
Array ( [0] => SQLSTATE[3D000]: Invalid catalog name: 1046 No database selected )
This is the code: (/register.php)
try{
$input_password_hash = password_hash($input_password, PASSWORD_DEFAULT);
$stmt = $PDO_new->prepare("SELECT user_name FROM users WHERE user_name = :username");
$stmt->bindParam(":username",$input_username);
$stmt->execute();
echo "code after exec";
//checks if user already exist in database.
if($stmt->rowCount()>0){
$error[] = "Username already exist";
echo "user is there";
}
else{
echo "user not there";
$insert = "INSERT INTO users(user_name, user_password_hash, user_email) VALUES($input_username,$input_password_hash, $input_email)";
}
} catch(PDOException $e){
$error[] = $e->getMessage();
}
This is the file for connecting to the databse: ((config/db_connect.php)
<?php
require_once($_SERVER['DOCUMENT_ROOT']."/config/db.php");
$PDO_new = new PDO("mysql:host=" . DB_HOST . ";DBName=" . DB_NAME, DB_USER, DB_PASS);
$PDO_new->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
And this is the file where I define the info for the database: (/config/db.php)
define("DB_HOST", "localhost");
define("DB_NAME", "username_databasename");
define("DB_USER", "username_admin");
define("DB_PASS", "password");

Answer 1

A quick test reveals that a PDO connection is case-sensitive in how it parses attributes. You use DBName when you should be using the lowercase dbname. This is not explicitly stated in the PDO::__construct() docs, as far as I can tell, but I was able to verify it testing in my own environment.
$PDO_new = new PDO("mysql:host=" . DB_HOST . ";dbname=" . DB_NAME, DB_USER, DB_PASS);
//--------------------------------------------^^^^^^^^^^
It is unclear why you are also establishing a connection via MySQLi, as MySQLi and PDO are comparable but incompatible APIs. You generally need one or the other, not both (unless you are combining code from 2 sources that use opposite APIs).
It is recommended to wrap the new PDO() in a try/catch to catch connection errors before setting ERRMODE_EXCEPTION on it.
try {
$PDO_new = new PDO("mysql:host=" . DB_HOST . ";dbname=" . DB_NAME, DB_USER, DB_PASS);
} catch (PDOException $e) {
// handle a connection error error
}
Finally, I would be remiss if I did not point out that you have plain variables in the INSERT statement rather than bound parameters. You should be binding values with placeholders here as you did in the first SELECT statement.
// Use placeholders please!
$insert = "INSERT INTO users(user_name, user_password_hash, user_email) VALUES(:input_username,:input_password_hash, :input_email)";
// prepare() then execute()...