1. Introduction

This tutorial is designed to guide you on how to handle file uploads securely in PHP. We'll cover how to receive a file, validate it, and store it safely on your server. By the end of this tutorial, you will be able to build a secure file upload system that prevents malicious file uploads, and ensures only valid files are stored.

You will learn:
- The basics of handling file uploads in PHP
- How to validate file types and sizes
- How to prevent malicious file uploads
- How to store uploaded files securely

Prerequisites:
- Basic understanding of PHP and HTML
- A PHP development environment

2. Step-by-Step Guide

File uploads in PHP involves three steps: receiving the file, validating it, and storing it.

Receiving the File: HTML form is used to receive the file from the user. The 'enctype' attribute should be set to 'multipart/form-data' and the form method should be 'POST'.

Validating the File: This involves checking the file type and size to ensure they meet your criteria. PHP's $_FILES array can be used to access file details.

Storing the File: Once validated, the file can be moved from temporary storage to your desired location using the move_uploaded_file() function.

Best Practices and Tips:
- Always validate the file type and size
- Avoid storing files in your web root directory
- Use unique names for your uploaded files to prevent overwrites

3. Code Examples

Example 1: Basic File Upload

<?php
if($_SERVER['REQUEST_METHOD'] == 'POST'){
    // Get the file details
    $file = $_FILES['userfile'];

    // Validate file size (max 2MB)
    if($file['size'] > 2000000){
        echo "File size is too large.";
        exit;
    }

    // Validate file type (only allow jpg, png, gif)
    $allowed_types = ['image/jpeg', 'image/png', 'image/gif'];
    if(!in_array($file['type'], $allowed_types)){
        echo "Invalid file type.";
        exit;
    }

    // Store the file
    $destination = '/path/to/your/storage/' . uniqid();
    move_uploaded_file($file['tmp_name'], $destination);
    echo "File uploaded successfully.";
}
?>

<!-- The HTML form for file upload -->
<form method="POST" enctype="multipart/form-data">
    <input type="file" name="userfile">
    <input type="submit" value="Upload">
</form>

Expected Result: The file is uploaded to your specified location if it meets the size and type criteria.

4. Summary

In this tutorial, we've covered how to securely handle file uploads in PHP. We've learnt how to receive, validate, and store uploaded files. The next step would be to integrate this knowledge into your web application.

Additional Resources:
- PHP Official Documentation on File Uploads
- W3Schools PHP File Upload Tutorial

5. Practice Exercises

Exercise 1: Create a file upload form that only accepts PDF files of size less than 1MB.

Exercise 2: Create a file upload form that accepts multiple files at once.

Exercise 3: Extend the file upload system to store uploaded files in separate directories based on their types.

Tips for Further Practice:
- Try to handle file upload errors
- Experiment with different file types and sizes
- Implement a file download functionality to the uploaded files.