<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
  <book id="1">
    <title>Learning XML</title>
    <author>John Doe</author>
    <price>29.99</price>
  </book>
</bookstore>

let name = "John";      // String
let age = 25;          // Number
let isStudent = true;  // Boolean
let x;                 // undefined
const PI = 3.14;       // constant
// Template literal
let msg = `Hello, ${name}!`;

// Selecting elements
document.getElementById("myBtn");        // by id
document.getElementsByTagName("p");      // by tag
document.querySelector(".class");       // first match
document.querySelectorAll("div.box");   // all matches

// Manipulation
element.innerHTML = "New content";
element.style.color = "red";

// Events
button.addEventListener("click", () => {
  alert("Clicked!");
});

document.getElementById("myForm")
  .addEventListener("submit", function(event) {
    event.preventDefault(); // stop default submit
    let name = document.getElementById("name").value;
    if (name.length < 2) {
      alert("Name must be ≥ 2 chars");
      return;
    }
    alert("Form valid! Submitting...");
  });

public class Student implements java.io.Serializable {
  private String name;
  private int age;
  public Student() {} // no-arg constructor required
  public String getName() { return name; }
  public void setName(String n) { name = n; }
}

<?php
$name      = "John";   // String
$age       = 25;       // Integer
$height    = 5.8;      // Float
$isStudent = true;    // Boolean

// Concatenation uses dot (.)
echo "Name: " . $name;

// if-elseif-else
if ($score >= 90) {
  echo "Grade: A";
} elseif ($score >= 80) {
  echo "Grade: B";
} else {
  echo "Grade: F";
}
?>

<?php
if ($_POST) {
  $username = $_POST['username'];
  $email    = $_POST['email'];

  // Sanitize output
  echo htmlspecialchars($username);

  // Validation
  if (empty($username)) {
    echo "Username required";
  }
  if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
    echo "Invalid email";
  }
}
?>

<?php
$conn = mysqli_connect("localhost","root","","college");
if (!$conn) {
  die("Connection failed: " . mysqli_connect_error());
}
echo "Connected successfully";
// ... do work ...
mysqli_close($conn); // ALWAYS close!
?>

$sql = "CREATE DATABASE college";
if (mysqli_query($conn, $sql)) {
  echo "DB created!";
}

mysqli_select_db($conn, "college");
// OR pass db name in connect()
mysqli_connect("h","u","p","college");

// A) SELECT — Fetch Data
$sql = "SELECT * FROM students";
$result = mysqli_query($conn, $sql);
while ($row = mysqli_fetch_assoc($result)) {
  echo $row["name"] . " - " . $row["age"];
}

// B) INSERT — Add Data
$sql = "INSERT INTO students (name,age) VALUES ('Amit',21)";
mysqli_query($conn, $sql);

// C) UPDATE — Modify Data
$sql = "UPDATE students SET age=22 WHERE name='Amit'";
mysqli_query($conn, $sql);

// D) DELETE — Remove Data
$sql = "DELETE FROM students WHERE name='Amit'";
mysqli_query($conn, $sql);

-- Add Column
ALTER TABLE students ADD email VARCHAR(50);

-- Modify Column
ALTER TABLE students MODIFY age INT(3);

-- Drop Column
ALTER TABLE students DROP email;

-- Template: ALTER TABLE tbl [ADD|MODIFY|DROP] col_name [TYPE]