<?php
    session_start();
        $id = $_SESSION['user_id'];
        $pic = $_FILES['pic']["name"];
        $folder = "../images/users/";
        move_uploaded_file($_FILES['pic']["tmp_name"], "$folder".$pic);

        $mysqli = connectDB();
        upload($id,$pic,$mysqli);
    ?>

function upload($id,$pic,$mysqli)
    {
        $pic = $mysqli->real_escape_string($pic);

        $sql = "UPDATE users SET pic = '$pic' WHERE id = '$id'";
        $result = $mysqli->query($sql);
    }

<form name="submit" action="include/upload.php" method="post">
    <input type="file" value="Choose file" accept="image/*" id="pic" name="pic">
    <input type="submit" value="Upload" name="submit">
</form>