Select students with

gmail

account. Using

IF

statement:

SELECT * 
FROM `students` 
WHERE IF(LENGTH(mail), mail, mail2) LIKE '%gmail.com%'
ORDER BY points

Using

OR

logical operator:

SELECT *
FROM  `students` 
WHERE 
    (`mail` LIKE '%gmail.com%' AND LENGTH(mail))
    OR 
    (mail2 LIKE '%gmail.com%') 
ORDER BY points

Using

CASE

SELECT * 
FROM `students` 
WHERE 
    (CASE
        WHEN LENGTH(mail) THEN mail
        ELSE mail2
    END) LIKE '%gmail.com%'
ORDER BY points

The

install

command reads the

composer.json

file from the current directory, resolves the dependencies, and installs them into

vendor

composer install  /  composer i

If

composer.lock

file exists, installs exactly what's specified in this file Otherwise 1. Reads

composer.json

file to look out what dependencies needs to be installed 2. Writes the composer.lock with the information of the project (installed dependencies)

Only

major

version will stick to

1

but

minor

and

patch

will be the latest versions

"dependencies": {
    "moment": "^1.6.2"
}

Only updates

patch

but

major

and

minor

will be the specified versions

"dependencies": {
    "moment": "~1.6.2"
}

Exactly the specified version

1.6.2

will be installed

"dependencies": {
    "moment": "1.6.2"
}

The latest version of the package will be installed

"dependencies": {
    "moment": "*"
}

element.

childNodes

returns an array of an element's child nodes. JS

function setText() {
    let a = document.getElementById("demo");
     let arr = a.childNodes;
     for(let x=0;x<arr.length;x++) {
       arr[x].innerHTML = "new text";
     }
}

//calling the function with setTimeout to make sure the HTML is loaded
setTimeout(setText, 500);

HTML of the example

<div id ="demo">
  <p>some text</p>
  <p>some other text</p>
</div>

Note: The code above changes the text of both paragraphs to

new text

This code creates a new paragraph and adds it to the existing div at the end of its content. JS

//calling the function in window.onload to make sure the HTML is loaded
window.onload = function() {

    //creating a new paragraph
    var p = document.createElement("p");
    p.innerHTML = 'Some new text 1';
       
    //adding the paragraph to the div
    var div = document.getElementById("demo"); 
    div.appendChild(p);
};

HTML

<div id="demo">some content</div>

getElementsByTagName

method returns all of the elements of the specified tag name as an array. The following example gets all paragraph elements of the page and changes their content. JS

let arr = document.getElementsByTagName("p");
for (let x = 0; x < arr.length; x++) {
  arr[x].innerHTML = "Hi there";
}

HTML of the example

<p>Hi</p>
<p>Hey</p>
<p>Hello</p>
<div>Hello there</div>

Note: We used the

length

property of the

array

to loop through all the selected elements in the above example

At first we need to create two

datetime

objects using

createFromFormat

static method of

DateTime

built-in class

$first_date = DateTime::createFromFormat('Y-m-d', "2020-12-23");
$second_date = DateTime::createFromFormat('Y-m-d', "2020-12-30");

Using

diff()

method of

DateTime

class we actually calculate differences between the two dates and we get the result in days

$difference_between_the_days = $second_date->diff($first_date)->format("%a");
	
echo $difference_between_the_days;

class Foo
{
    public static $staticVariable = 'foo';

    public static function getVariableStatic() {
        return self::$staticVariable;
    }

    public function getVariableNONStatic() {
        return self::$staticVariable;
    }
}

We can not access

$staticVariable

using an object directly:

$foo = new Foo();
print $foo->staticVariable;

This will produce the following error message:

Notice:  Accessing static property Foo::$staticVariable as non static...

But there are several ways to access static variable using an object: 1. Using static getter method:

$foo = new Foo();
print $foo->getVariableStatic();

2. Using non-static getter method:

$foo = new Foo();
print $foo->getVariableNONStatic();

php.net:

A property declared as static cannot be accessed with an instantiated class object (though a static method can).

Type hinting means to define which class object we want to receive exactly as a parameter in a constructor

class BLock { }
class Lock {
    private $isLocked;
    
    public function __construct() {
        
    }
    
    public function lock() {
        $this->isLocked = true;
    }
    
    public function unLock() {
        $this->isLocked = false;
    }
    
    public function isLocked() {
        return $this->isLocked;
    }
}
class Chest {
    private $lock;
    
    public function __construct(Lock $lock) {
        $this->lock = $lock;
    }
    
    public function close() {
        $this->lock->lock();
        echo 'Closed' . PHP_EOL;
    }
    
    public function open() {
        if ($this->lock->isLocked()) {
            $this->lock->unLock();
        }
        echo 'Opened' . PHP_EOL;
    }
    
    public function isClosed() {
        return $this->lock->isLocked();
    }
}

$chest = new Chest(new Lock);
// $chest = new Chest(new Block);

$chest->open();
$chest->close();

In this example

Chest

class constructor waits to receive

Lock

class object. If we pass any other class object, we will get an error like the following: Fatal error:

Uncaught TypeError: Argument 1 passed to Chest::__construct() must be an instance of Lock, instance of BLock given

class

- blueprint, skeleton, basic structure of an object

property

- data of an object, equivalent to PHP variables

method

- behavior of an object, equivalent to PHP functions

inheritance

- ability to use properties and methods of an existing class

polymorphism

- ability to have many forms, when a class has varying functionality while sharing a common interfaces

encapsulation

- to hide or protect certain properties or methods of an object

abstraction

- a concept in which a class has methods without implementation