If we use several

traits

in the same class, the traits must not have the same method declared

trait Laser {
    public function who() { 
        echo 'I am a Laser' . PHP_EOL; 
    }
}
trait Projector {
    public function who() { 
        echo 'I am a Laser' . PHP_EOL; 
    }
}
class Galaxy {
    use Laser;//, Projector;
    use Projector;
}

Fatal error:

Trait method who has not been applied, because there are collisions with other trait methods on Galaxy

getmypid

- Gets PHP's process ID

echo 'Current process ID:' . getmypid();

The output will be:

Current process ID:4868

Content of

index.php

file

spl_autoload_register(function($class) {
    // echo 'register class:'.$class."<br>";
    require_once("classes/{$class}.php");
});

// echo 'hey there on line 7'."<br>";

$cat = new Cat;
$dog = new Dog;
$tortoise = new Tortoise;

echo $cat->talk();
echo $dog->talk();
echo $tortoise->talk();

Content of

classes/Talkative.php

file

interface Talkative {
    public function talk();
}

Content of

classes/Cat.php

file

class Cat implements Talkative {
    public function talk() {
        return 'Meow' . '<br>';
    }
}

Content of

classes/Dog.php

file

class Dog implements Talkative {
    public function talk() {
        return 'Woof' . '<br>';
    }
}

Content of

classes/Tortoise.php

file

class Tortoise implements Talkative {
    public function talk() {
        return 'Yak yak yak yak ...' . '<br>';
    }
}

Filenames and class names must be THE SAME

Makes our code more modular and reusable

Makes our code easier to maintain

Makes it easier to debug when things go wrong

Makes it possible to hide / protect CLASS properties / methods (encapsulation)

Makes it possible to use other class properties / methods (inheritance)

Makes it possible to use polymorphism

Shows all packages without their dependencies (only top level)

npm list --depth 0

Shows all packages with only their dependencies (one level deep)

npm list --depth 1

Variable

lastName

declared by the keyword

let

is not added to the global

window

object. That is why the variable is equal to

undefined

// Declaring variable using keyword "var"
var firstName = "Jet"

// Declaring variable using keyword "let"
let lastName = "Li"

// Declaring variable without using any keyword
carMark = "Tesla"

function driveCar() {
  console.log(this.firstName + ' ' + this.lastName + ' is driving a car '+this.carMark)
}

driveCar()

The result will be:

Jet undefined is driving a car Tesla

Because declaring variable with

var

keyword and without any declaration keyword are the same. Both add the variable to the global

window

object. After running the above code, global object

window

will have

firstName

and

lastName

attributes

window.firstName;  // "Jet"
window.lastName;  // undefined
window.carMark;  // "Tesla"

we can change the attributes of elements. For example, we can change the src attribute of an image. JS

var el = document.getElementById("myimg");
el.src = "https://www.sololearn.com/Images/home-new/1073.png";

HTML

<img id="myimg" src="https://www.sololearn.com/Images/home-new/1068.png" alt="" />

Note: Practically all

attributes

of an element can be changed using

JavaScript

Regular anonymous function:

// Anonymous function
document.addEventListener('click', function(){
  console.log('You clicked me!');
})

The same function with arrow function syntax

// Arrow function
document.addEventListener('click', () => {
  console.log('You clicked me!');
})

Even more minimalistic arrow function

// More minimalistic arrow function 
document.addEventListener('click', () => console.log('You clicked me!'))

CROSS JOIN

is the same as

INNER

or

implicit

joins. Without conditions, both result in Cartesian product:

SELECT 
    students.*,
    notes.*
FROM `notes`
CROSS JOIN `students` 

JOIN

example:

SELECT 
    students.*,
    notes.*
FROM `notes`
JOIN `students` 

JOIN

is the short form of

INNER JOIN

SELECT 
    students.*,
    notes.*
FROM `notes`
INNER JOIN `students` 

Implicit

join example

SELECT 
    students.*,
    notes.*
FROM `notes`, `students` 

Both the queries return the exact same result

Registers a function that will be executed right before the PHP process stops execution

register_shutdown_function('myexit');

function myexit()
{
	echo "</schedule>";
}