Checks if user already exists. If not, it inserts the user only one time.

check_user.php

<?php
	
	try {
		$con = new PDO("mysql:host=localhost;","root","");
		$con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);	
		
		$username = "John Doe"; 
		$password = "abc123";
		
		// CREATE
		$con->exec("
			CREATE DATABASE IF NOT EXISTS registered_users;
			USE registered_users;
			CREATE TABLE IF NOT EXISTS users (
				username VARCHAR(31) NOT NULL,
				password VARCHAR(31) NOT NULL
			);
		");
		
		// INSERT IF NOT EXISTS
		$stmt = $con->prepare("
			INSERT INTO users (
				username, password
			)
			SELECT * FROM (
				SELECT :username,
				:password
			) 
			AS compare
			WHERE NOT EXISTS (
				SELECT username 
				FROM users 
				WHERE username = :username
				AND password = :password
			) LIMIT 1;
		");
		$taken = $stmt->rowCount();
		$stmt->bindParam(":username", $username);
		$stmt->bindParam(":password", $password);
		if ($stmt->execute()) {
			$pfx = "<p>Username <strong>".$username."</strong> ";
			if ($stmt->rowCount() == $taken) {
				echo $pfx."is taken.</p>";
			} else {
				echo $pfx."was added.</p>";
			}
		}
		
		
		// SELECT
		$show = $con->prepare("
			SELECT * FROM users
		");
		if ($show->execute()) {
			$result = $show->setFetchMode(PDO::FETCH_OBJ);
			foreach($show->fetchAll() as $data) {
				echo $data->username." | ".$data->password."<br>";
			}
		}
		
		// DELETE
		$del = $con->prepare("
			DELETE FROM users
		");
		/*if ($del->execute()) {
			echo "Table has been cleared.";
		}*/
		
	}
	catch(PDOException $e) {
		echo "Error: " . $e->getMessage();
	}
	$con = null;
	
?>