.class1 .class2 h2{
    color: red;
}

Selects all

<h2>

elements within

.class2

that is a descendant of

.class1

element

The text inside

p

element will be all uppercase and underlined

p {
    text-decoration: underline;
    text-transform: uppercase;
}

Child class can not override final method of the parent class

class A {
    final public static function who() {
        echo __CLASS__;
    }
}

class B extends A {
    public static function who() {
        echo __CLASS__;
    }
}

B::who();

Fatal error:

Cannot override final method A::who()

An element with position: sticky; is positioned based on the user's scroll position

div.sticky {
  background-color: yellow;
  position: sticky;
  top: 0;
}

A sticky element toggles between relative and fixed, depending on the scroll position. It is positioned relative until a given offset position is met in the viewport - then it "sticks" in place like

position:fixed

Selects all the

p

elements that have

div

element as a parent of any level

div p { 
    color: red; 
}

Selects all the

p

elements that are children of the element with id

#id1

#id1 p { 
    color: green; 
}

Selects all

p

elements that have parent element with class

.class1

.class1 p { 
    color: yellow; 
}

To replace an HTML element, the element.

replaceChild

(newNode, oldNode) method is used. JS

window.onload = function() {
    let p = document.createElement("p");
    p.innerHTML = "This is new";

    // Replaces the first p with the new one
    let parent = document.getElementById("demo");
    parent.replaceChild(p, document.getElementById("p1"));
};

HTML

<div id="demo">
	<p id="p1">This is a paragraph.</p>
	<p id="p2">This is another paragraph.</p>
</div>

Note: The code above creates a new paragraph element that replaces the existing

p1

paragraph

To remove an HTML element, we can select the parent of the element and use the

removeChild

(node) method JS

window.onload = function() {
    var parent = document.getElementById("demo");
    parent.removeChild(document.getElementById("p1"));
};

HTML

<div id="demo">
    <p id="p1">This is a paragraph.</p>
    <p id="p2">This is another paragraph.</p>
</div>

element.

insertBefore

(node1, node2) inserts

node1

as a child before

node2

JS

function myFunction() {
  // Creates new list item
  let newItem = document.createElement("li");
  newItem.innerHTML = "Water";
  
  // Puts list item into myList before "tea" item
  let list = document.getElementById("myList");
  list.insertBefore(newItem, document.getElementById("tea"));
}

HTML

<ul id="myList">
  <li>Coffee</li>
  <li id="tea">Tea</li>
  <li>Cocacola</li>
</ul>

<p>Click the button to insert an item to the list.</p>

<button onclick="myFunction()">Try it</button>

Note: If the function parameter is not surrounded by

quotation marks

, then the parameter is going to be interpreted as a column value. Converts a string or column value to

UPPER-CASE

SELECT 
    UPPER(first_name), 
    UPPER('Converts a string or column value to UPPER-CASE') 
FROM students

Converts a string or column value to

lower-case

SELECT 
    LOWER(first_name), 
    LOWER('Converts a string or column value to lower-case') 
FROM students

Joins strings and columns together

SELECT 
    CONCAT(first_name, ' ', 'joins', ' ', 'strings', ' ', 'and', ' ', 'columns') 
FROM students

The function

LENGTH

Returns the length (in bytes)

SELECT 
    id, 
    first_name,
    LENGTH(first_name) first_name_length 
FROM students

Note: Each Georgian letter takes

three bytes

The function is similar to

CONCAT

where

WS

means - With Separator. The first parameter is the separator between each one of the additional fields, that we pass as next parameters

SELECT 
    CONCAT_WS(' - ', first_name, 'joins', 'strings', 'and', 'columns', 'with', 'separator') 
FROM students

The function

TRIM

removes leading and trailing spaces

SELECT 
    LENGTH(' text ') length,
    LENGTH(TRIM(' text ')) length_with_trim

The function

RTRIM

removes trailing spaces (removes spaces from the end)

SELECT 
    LENGTH(' text ') length,
    LENGTH(RTRIM(' text ')) length_with_right_trim

The function

LTRIM

remove leading spaces (removes spaces from the beginning)

SELECT 
    LENGTH(' text ') length,
    LENGTH(LTRIM(' text ')) length_with_left_trim

The function

LEFT

returns leftmost characters. In this case 5 characters because we pass 5 as second parameter

SELECT 
    LEFT(first_name, 5) AS 'five leftmost characters'
FROM students

The function

RPAD

appends string (third parameter) the specified number of times (second parameter) to the first parameter. In this example each one of the student's last name that is less than 10 characters long, is filled with

-

SELECT 
    RPAD(first_name, 10, '-') AS 'student name'
FROM students

Complete list of string functions on the official documentation: https://dev.mysql.com/doc/refman/8.0/en/string-functions.html

Two classes extends

Animal

abstract class and both implement the same method

prey()

differently

abstract class Animal {
    // child classes must implement this
    abstract function prey();

    public function run() {
        echo "I am running!\n";
    }
}

class Dog extends Animal {
    public function prey() {
        echo "I killed the cat !\n";
    }
}

class Cat extends Animal {
    public function prey() {
        echo "I killed the rat !\n";
    }
}

$dog = new Dog();
$cat = new Cat();

$dog->prey(); // I killed the cat !
$cat->prey(); // I killed the rat !

$dog->run(); // I am running!
$cat->run(); // I am running!