process_submission.php

<?php
session_start();
require "config.php";

if (!isset($_SESSION['id']) || $_SESSION['role'] !== 'student') {
    die("Access denied");
}

$user_id = $_SESSION['id'];
$assignment_id = intval($_POST['assignment_id'] ?? 0);

if (!isset($_FILES['submission_file'])) {
    die("No file uploaded");
}

$file = $_FILES['submission_file'];
if ($file['error'] !== UPLOAD_ERR_OK) {
    die("Upload error");
}

// validate size (10MB)
if ($file['size'] > 10 * 1024 * 1024) {
    die("File too large");
}

// validate extension
$allowed = ['pdf','doc','docx','zip','rar'];
$ext = strtolower(pathinfo($file['name'], PATHINFO_EXTENSION));
if (!in_array($ext, $allowed)) {
    die("Invalid file type");
}

// prepare destination
$dir = __DIR__ . "/uploads/assignments/submissions/";
if (!is_dir($dir)) mkdir($dir, 0755, true);

$filename = time() . "_" . preg_replace('/[^A-Za-z0-9._-]/', '_', basename($file['name']));
$destPath = $dir . $filename;

if (!move_uploaded_file($file['tmp_name'], $destPath)) {
    die("Move failed");
}

// store in DB: assignment_submissions (id, assignment_id, user_id, filename, filepath, uploaded_at)
$filepath_db = "uploads/assignments/submissions/" . $filename;
$stmt = $pdo->prepare("
    INSERT INTO assignment_submissions (assignment_id, user_id, filename, filepath, uploaded_at)
    VALUES (?, ?, ?, ?, NOW())
");
$stmt->execute([$assignment_id, $user_id, $file['name'], $filepath_db]);

// redirect back to dashboard or assignment page
header("Location: student_dashboard.php?submission=success");
exit;