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Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, null given in C:wampwwwphpfunctions.php on line 10

<?php
    session_start();

    require_once 'connect.php';
    include 'functions.php';

    getData();
?>

<?php
    $servername = "localhost";
    $username = "username";
    $password = "password";

    $conn = mysqli_connect("$servername","$username","$password") or die ("could not connect to mysql");

    mysqli_select_db($conn,"php") or die ("Database not found.");

    function kill(){
        mysqli_close($conn);
        header('Location:/login');
    }
?>

<?php

    require_once 'connect.php';

    function getData() {
        if(isset($_SESSION['username'])){
            $username = $_SESSION['username'];
            $query = "SELECT first_name FROM users WHERE user_name = '$username'";
            $result = mysqli_query($conn, $query);
            $userdata=mysqli_fetch_array($result);
            echo $userdata["first_name"];
    }
}