Challenge 3 — Solution

Task: Write a complete safe file-upload handler for a field named
"document" that only allows PDF files (application/pdf) up to 5MB,
checks the real MIME type with mime_content_type(), generates a new
filename with uniqid(), and moves the file into an "uploads/" folder —
echoing clear success or failure messages at each validation step.

<?php
    $allowedTypes = ['application/pdf'];
    $maxSize = 5 * 1024 * 1024;   // 5 MB

    if ($_FILES['document']['error'] !== UPLOAD_ERR_OK) {
        echo "Upload failed — please try again.<br>";
    } elseif ($_FILES['document']['size'] > $maxSize) {
        echo "File too large (max 5MB).<br>";
    } else {
        $actualType = mime_content_type($_FILES['document']['tmp_name']);

        if (!in_array($actualType, $allowedTypes)) {
            echo "Invalid file type — only PDF files are allowed.<br>";
        } else {
            $newName = uniqid() . '.pdf';
            $destination = "uploads/$newName";

            if (move_uploaded_file($_FILES['document']['tmp_name'], $destination)) {
                echo "Document uploaded successfully as $newName.<br>";
            } else {
                echo "Something went wrong while saving the file.<br>";
            }
        }
    }
?>


Output:
(Depends on the actual uploaded file - assuming a genuine, valid PDF
under 5MB was submitted:)
Document uploaded successfully as 65432a1b8f9c3.pdf.

Notes:
- The four checks run in a deliberate order: upload error first (did
  the upload even succeed at the transport level), then size, then the
  real content type via mime_content_type() - each elseif only runs if
  every earlier check already passed, avoiding wasted work checking a
  file that already failed an earlier, cheaper test.
- mime_content_type() is used specifically instead of trusting
  $_FILES['document']['type'], since the claimed type is set by the
  browser and can be faked - a file renamed to look like a PDF but
  containing something else entirely would be caught here.
- uniqid() . '.pdf' generates a completely new, unpredictable filename,
  discarding the visitor's own original filename entirely - preventing
  both filename collisions between different uploads and the path-
  traversal risk of trusting a visitor-supplied filename directly.
- move_uploaded_file()'s own return value is checked too (rather than
  assuming it always succeeds), since a genuine filesystem-level problem
  (e.g. the uploads/ folder not being writable) is still possible even
  after every earlier validation step has passed.