PHP/MySQL: Check if username exists - php

Question

I'm a beginner in php and I want to check if the username entered already exists.
Here is my code.
<?php
ini_set('display_errors',1);
error_reporting(E_ALL);
if (isset($_POST['submit'])) {
include "connect.php";
ValidateUser();
}
function ValidateUser()
{
if (!empty($_POST['username']) AND !empty($_POST['password'])) {
$queryrow=mysqli_query("SELECT * FROM websiteusers WHERE username = '$_POST['username']'");
if ($rows=mysqli_num_rows($queryrow)=0) {
RegisterUser();
}
}
function RegisterUser() {
echo "works up to here";
}
?>
It doesn't even give me an error despite turning error reporting on.

Answer 1

Have you even initialized a mysqli_connect?
$Connection = mysqli_connect("host","user","pass","database");
Then pass it to a function which uses mysqli_query() by:
function foo ($DB){
mysqli_query($DB,"QUERY HERE");
// Do other stuff
return /* Whatever you wish to return here*/
}
foo($Connection);

Answer 2

What you are trying to achieve can be done very easily with the following code. A bigger concern is security. It is good practice to both sanitize your input every time the user has a chance to input text.
Also, using prepared query's will put yet another layer of security.
Although this isn't using your provided code directly, I believe it is good to teach good habits.
If you have any questions feel free to ask.
$username = $_POST['username']; <-- sanitize this
$message = null;
$mysqli = new mysqli("localhost", "user", "password", "database");
$stmt = $mysqli->prepare("SELECT username FROM websiteusers WHERE username=?");
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($usernamesql);
$stmt->fetch();
if ($stmt->num_rows() > 0) {
RegisterUser();
} else {
$message .= 'username already exists';
}
Later on when you require more items to be queried, or more results to be bound:
$stmt = $mysqli->prepare("SELECT username,password,other1,other2 FROM websiteusers WHERE username=?");
$stmt->bind_param('s', $username); <-- the "s" means the argument is a strings, if a argument is stored as an int use "i", but one character for each argument is required.
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($usernamesql);
$stmt->fetch();
Multiple Arguments:
$stmt = $mysqli->prepare("SELECT username,password,other1,other2 FROM websiteusers WHERE username=? AND authenticated=?");
$stmt->bind_param('si', $username,$isauthenticated); <-- second argument is a INT or BOOL
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($usernamesql,$passwordsql,$other1sql,$other2sql);
$stmt->fetch();
When your expecting multiple results, and lets say you want to dump them into arrays:
$userarray = array();
$stmt = $mysqli->prepare("SELECT username FROM websiteusers WHERE username=?");
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($usernamesql);
while($stmt->fetch()){
array_push($userarray, $usernamesql);
}
$userarray is now an array of all the results fetched from the database.

Answer 3

Here is the right way to do this:
<?php
ini_set('display_errors',1);
error_reporting(E_ALL);
if (isset($_POST['submit'])) {
$mysqli = new mysqli("localhost", "user", "password", "database");
if ($mysqli->connect_errno) {
echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}
if(check_user($mysqli, $_POST['username']){
registerUser();
}else{
echo 'user exist, cannot register';
}
}
function check_user($conn, $username){
$query = "SELECT * FROM websiteusers WHERE username = ?";
if ($stmt = $conn->prepare($query)) {
$stmt->bind_param("s", $username);
$stmt->execute();
$stmt->close();
}
return $stmt->num_rows === 0;
}
function registerUser() {
echo "registering user ...";
}
Read up on prepared statement