Script for both logging into or registering an account on a website.

login-register.php

<?php
if (isset($_POST['login']) && $_POST['login']) {
	if (!isset['username'] && !isset['email']) {
		echo 'No username or email provided!';
		exit(1);
	}
	if (!isset['passwd']) {
		echo 'No password provided!';
		exit(1);
	}
	if (!($con = new mysqli('localhost', 'mysql_user', 'mysql_pass', 'users'))) {
		echo 'Could not connect to database!';
		exit(2);
	}
	$qry = $con->prepare('SELECT * FROM accounts WHERE ' . ((isset($_POST['username'])) ? 'user' : 'email') . ' = ?';
	if (!$qry->bind_param('s', ((isset($_POST['username'])) ? $_POST['username'] : $_POST['email']))) {
		echo 'Could not query database!';
		exit(2);
	}
	if (!$qry->execute()) {
		echo 'Could not query database!';
		exit(2);
	}
	$qry->bind_result($account);
	if (($res = $qry->fetch()) === null) {
		echo 'User does not exist!';
		exit(3);
	}
	if (!$res) {
		echo 'Could not query database!';
		exit(2);
	}
	if (hash("sha256", $_POST['passwd'] . $account['salt']) != $account['passwd']) {
		echo 'Incorrect password!';
		exit(3);
	}
	$qry = $con->prepare('UPDATE accounts SET last_login = NOW() WHERE ' . ((isset($_POST['username'])) ? 'user' : 'email') . ' = ?')
	if (!$qry->bind_param('s', ((isset($_POST['username'])) ? $_POST['username'] : $_POST['email']))) {
		echo 'Could not update database!';
		exit(2);
	}
	if (!$qry->execute()) {
		echo 'Could not query database!';
		exit(2);
	}
	if (!$qry->affected_rows()) {
		echo 'Could not query database!';
		exit(2);
	}
	session_start();
	$_SESSION['uuid'] = $account['uuid'];
	$_SESSION['name'] = $account['user'];
	echo 'Logged in successfully!';
	exit(0);
} else {
	if (!isset($_POST['username']) || !isset($_POST['email'])) {
		echo 'No username or email provided!';
		exit(1);
	}
	if (!isset($_POST['passwd'])) {
		echo 'No password provided!';
		exit(1);
	}
	if (!($con = new mysqli('localhost', 'mysql_user', 'mysql_pass', 'users'))) {
		echo 'Could not connect to database!';
		exit(2);
	}
	$saltStr = '';
	$len = random_int(16, 32);
	for($i = 0; $i < $len; $i++) $saltStr .= 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_'[random_int(0, 63)];
	$saltStr = hash("sha256", $saltStr);
	$passwd = hash("sha256", ($_POST['passwd'] . $saltStr));
	$qry = $con->prepare("INSERT INTO accounts (user, email, passwd, salt) VALUES (?, ?, ?, ?)");
	if (!$qry->bind_param('ssss', $_POST['username'], $_POST['email'], $passwd, $saltStr)) {
		echo 'Could not query database!';
		exit(2);
	}
	if (!$qry->execute()) {
		echo 'Could not query database!';
		exit(2);
	}
	if (!$qry->affected_rows()) {
		echo 'Could not query database!';
		exit(2);
	}
	echo 'Account created successfully!';
	exit(0);
}