HTML PHP - How to check if the username already existed - php

Question

The script is already working fine but I want to insert a command that allows only if the username is not yet used.
if (isset($_POST['submit'])) {
$firstname = htmlentities($_POST['firstname'], ENT_QUOTES);
$lastname = htmlentities($_POST['lastname'], ENT_QUOTES);
$position = htmlentities($_POST['position'], ENT_QUOTES);
$username = htmlentities($_POST['username'], ENT_QUOTES);
$password = htmlentities($_POST['password_two'], ENT_QUOTES);
$uniqid = uniqid('', true);
if ( $firstname == '' || $lastname == '' || $position == '' || $username == '' || $password == '') {
$error = 'ERROR: Please fill in all required fields!';
renderForm($error, $firstname, $lastname, $position, $username, $password);
} else {
if ($stmt = $connection->prepare("INSERT INTO employee (uniqid, firstname, lastname, position, username, password) VALUES (?, ?, ?, ?, ?, ?)")) {
$stmt->bind_param("ssssss", $uniqid, $firstname, $lastname, $position, $username, $password);
$stmt->execute();
$stmt->close();
} else {
echo "ERROR: Could not prepare SQL statement.";
}
header("Location: regemployee.php");
}
} else {
renderForm();
}

Answer 1

Make username unique on the DB, then when you try to insert the same username in to the DB again, the insert will through an error.
Alternatively you could do a SELECT * FROM employee WHERE username = ? and check if results is > 0.
Then you would know it exists already.

Answer 2

Do another SELECT query which checks if the submitted username already exist:
$stmt = $connection->prepare("SELECT * FROM employee WHERE username = ?");
$stmt->bind_param("s", $username);
$stmt->execute();
Then get the number of results:
$stmt->store_result();
$noofres = $stmt->num_rows;
$stmt->close();
Then, create a condition that if it yet doesn't exist, it will do the insert query:
if($noofres == 0){
/* INSERT QUERY HERE */
} else {
echo 'Username already taken.';
}