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Valeri Tandilashvili's PHP Notes
Results: 135
call parent class constructor from child class CODE
We can call parent class constructor from child class using 
parent
keyword
parent::__construct($name);
Complete example:
class User {
    public $username;

    function __construct($name) {
        $this->username = $name;
    }
}
class Admin extends User{
    public $admin_level;
    
    function __construct($name, $admin_level) {
        parent::__construct($name);
        $this->admin_level = $admin_level;
    }
}
$admin1 = new Admin('Tom', 'Manager');

echo $admin1->username;
echo "\n";
echo $admin1->admin_level;
If there is no constructor in child class, then parent class constructor will be called (when appropriate parameters are passed)
preg_match
Checks username value against the rule: must be at least 6 characters long and at most 12 characters long, only allowed alphanumeric symbols
if (!preg_match('/^[a-zA-Z0-9]{6,12}$/', $username)) {
    $this->addError('username', 'username must be 6-12 chars & alphanumeric');
}
class implements several interfaces CODE
One class implements several interfaces and takes responsibility to define all methods from both interfaces. Also defines several private methods additionally
interface int1 {
    // Force Extending class to define these methods
    function getArea();
} 

interface int2 {
    function getPerimeter();
} 

class Circle implements int1, int2 {
    static $PI = 3.1415926536;
    public $radius;
    
    public function __construct($radius) {
        $this->radius = $radius;
    }

    public function getPI() {
        return self::$PI;
    }
    
    public function getArea() {
        return pow($this->radius, 2) * self::$PI;
    }
    
    public function getDiameter() {
        return $this->radius * 2;
    }
    
    public function getPerimeter() {
        return $this->radius * 2 * self::$PI;
    }
}

$circle1 = new Circle(5);
echo "\nPI: " . $circle1->getPI();
echo "\nArea: " . $circle1->getArea();
echo "\nDiameter: " . $circle1->getDiameter();
echo "\nPerimeter: " . $circle1->getPerimeter();
interface example CODE
In this example class 
Circle
implements 
CircleAbstract
interface. Therefore it must define 
getArea()
and 
getPerimeter()
methods 
interface CircleAbstract {
    // Force Extending class to define these methods
    function getArea();
    function getPerimeter();
} 

class Circle implements CircleAbstract {
    static $PI = 3.1415926536;
    public $radius;
    
    public function __construct($radius) {
        $this->radius = $radius;
    }

    public function getPI() {
        return self::$PI;
    }
    
    public function getArea() {
        return pow($this->radius, 2) * self::$PI;
    }
    
    public function getDiameter() {
        return $this->radius * 2;
    }
    
    public function getPerimeter() {
        return $this->radius * 2 * self::$PI;
    }
}

$circle1 = new Circle(5);
echo "\nPI: " . $circle1->getPI();
echo "\nArea: " . $circle1->getArea();
echo "\nDiameter: " . $circle1->getDiameter();
echo "\nPerimeter: " . $circle1->getPerimeter();
Inheritance example CODE
The child class extends parent class means that it borrows properties and methods from its parent
class User {
    public function მარტივიჯამი($num1,$num2) {
        return $num1 + $num2;
    }
}
class Admin extends User{
    
}
$admin1 = new Admin('Tom');

echo $admin1->მარტივიჯამი(4,5);
access modifiers / visibility modifiers CODE 
public
- visible from anywhere 
private
- visible only for the owner class 
protected
- visible only for the owner and it's child classes 
class User {
    public $username;
    public $friends = ['Tom','David'];

    function __construct($name) {
        $this->username = $name;
        print $this->username . "'s object is created'\n";
    }

    private function მარტივიჯამი($num1,$num2) {
        return $num1 + $num2;
    }
}
class Admin extends User{
    function sumNumbers($num1,$num2) {
        return $this->მარტივიჯამი($num1,$num2);
    }
}
$admin1 = new Admin('Tom');

echo $admin1->sumNumbers(4,5);
__clone method and clone function 
clone
function clones the object and 
__clone
method is going to call when the object gets cloned 
class User {
    public $username;
    public $friends = ['Tom','David'];

    function __construct($name) {
        $this->username = $name;
        print $this->username . "'s object is created'\n";
    }

    function __clone() {
        print $this->username . "'s object is cloned'\n";
    }
}
$user1 = new User('Tom');
$user2 = clone $user1;
abstract class can contain both methods with definitions and abstract methods 
abstract class Fruit { 
    private $color; 
    
    abstract public function eat(); 
    
    public function setColor($c) { 
        $this->color = $c; 
    } 
} 

class Apple extends Fruit {
    public function eat() {
        echo "Omnomnom";
    }
}

$obj = new Apple();
$obj->eat();
In this example an 
interface
would not be able to have method with definition, only method signatures are allowed in interfaces
calling non-static methods statically is deprecated 
class Foo {

    private $color;

    public function bar() {
        echo 'before';
        $this->color = "blue";
        echo 'after';
    }
}

// Foo::bar();  

// Deprecated
$obj = new Foo;
$obj::bar();
Deprecated: 
Non-static method Foo::bar() should not be called statically
php.net: 
In PHP 7, calling non-static methods statically is deprecated, and will generate an E_DEPRECATED warning. Support for calling non-static methods statically may be removed in the future
Guitar implements IMusician interface 
interface IMusician {
    public function play();
}

class Guitarist implements IMusician {
    public function play() { 
        echo "playin a guitar";
    }
}
In this example 
Guitarist
child class has to define 
play()
method because the child class implements interface 
IMusician
that has the method signature
Results: 135