Unit 3: Forms, files and database connectivity

php<?php
$errors = [];
if ($_SERVER["REQUEST_METHOD"] === "POST") {
    $name  = trim($_POST["name"] ?? "");
    $email = filter_var($_POST["email"] ?? "", FILTER_VALIDATE_EMAIL);
    $hobbies = $_POST["hobby"] ?? [];        // checkboxes named hobby[] arrive as an array
    if ($name === "") $errors[] = "Name is required";
    if (!$email)      $errors[] = "Invalid email";
    if (!$errors) echo "Thank you, " . htmlspecialchars($name);
}
?>
<form method="post" action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>">
  <input name="name"> <input name="email">
  <input type="checkbox" name="hobby[]" value="Music"> Music
  <input type="checkbox" name="hobby[]" value="Sports"> Sports
  <button>Submit</button>
</form>

• Sticky forms re-display entered values after an error; self-processing forms post to the same page.

php<?php
$f = fopen("students.txt", "a");
fwrite($f, "Aman,82\n");
fclose($f);

$f = fopen("students.txt", "r");
while (($line = fgets($f)) !== false) echo $line . "<br>";
fclose($f);

copy("students.txt", "backup.txt");          // copy
rename("backup.txt", "old/backup.txt");      // rename or move
if (file_exists("temp.txt")) unlink("temp.txt");   // delete
echo filesize("students.txt") . " bytes";
?>

php<?php
if (!is_dir("uploads")) mkdir("uploads", 0755);   // create
foreach (scandir("uploads") as $item) {             // list
    if ($item !== "." && $item !== "..") echo $item . "<br>";
}
$d = opendir("uploads");                            // older style
while (($entry = readdir($d)) !== false) echo $entry;
closedir($d);
rmdir("emptyfolder");                               // remove an empty directory
?>

• The GD library creates and edits images (PNG, JPEG, GIF) on the server — charts, thumbnails, CAPTCHAs, watermarks.

php<?php
header("Content-Type: image/png");
$img = imagecreatetruecolor(300, 120);
$bg  = imagecolorallocate($img, 240, 240, 255);
$blue = imagecolorallocate($img, 30, 60, 160);
imagefill($img, 0, 0, $bg);
imagerectangle($img, 5, 5, 294, 114, $blue);
imagestring($img, 5, 60, 50, "SBS Ludhiana", $blue);
imagepng($img);          // send to browser
imagedestroy($img);      // free memory
?>

php<?php
$conn = new mysqli("localhost", "root", "", "college");
if ($conn->connect_error) die("Connection failed: " . $conn->connect_error);

$result = $conn->query("SELECT roll, name, marks FROM student");
while ($row = $result->fetch_assoc()) {
    echo $row["roll"] . " " . $row["name"] . " " . $row["marks"] . "<br>";
}
$conn->close();
?>

php<?php
// INSERT with a prepared statement (prevents SQL injection)
$stmt = $conn->prepare("INSERT INTO student (name, marks) VALUES (?, ?)");
$stmt->bind_param("si", $name, $marks);          // s = string, i = integer
$stmt->execute();
echo "Inserted id " . $conn->insert_id;

// UPDATE
$stmt = $conn->prepare("UPDATE student SET marks = ? WHERE roll = ?");
$stmt->bind_param("ii", $newMarks, $roll);
$stmt->execute();
echo $stmt->affected_rows . " row(s) updated";

// DELETE
$stmt = $conn->prepare("DELETE FROM student WHERE roll = ?");
$stmt->bind_param("i", $roll);
$stmt->execute();

// SELECT with a condition
$stmt = $conn->prepare("SELECT name, marks FROM student WHERE marks >= ?");
$stmt->bind_param("i", $min);
$stmt->execute();
$rows = $stmt->get_result()->fetch_all(MYSQLI_ASSOC);
?>

• Tables: student(roll, name, course_id) and course(id, title); employee(id, name, manager_id).

sql-- CROSS JOIN: every student with every course (Cartesian product)
SELECT s.name, c.title FROM student s CROSS JOIN course c;

-- INNER JOIN: only matching rows
SELECT s.name, c.title FROM student s INNER JOIN course c ON s.course_id = c.id;

-- LEFT OUTER JOIN: all students, course if any
SELECT s.name, c.title FROM student s LEFT JOIN course c ON s.course_id = c.id;

-- RIGHT OUTER JOIN: all courses, students if any
SELECT s.name, c.title FROM student s RIGHT JOIN course c ON s.course_id = c.id;

-- SELF JOIN: employee with their manager
SELECT e.name AS employee, m.name AS manager
FROM employee e LEFT JOIN employee m ON e.manager_id = m.id;

• MySQL has no FULL OUTER JOIN; combine a LEFT and a RIGHT join with UNION.