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Valeri Tandilashvili's PHP Notes
Results: 133
filter_var (email validation)
Returns false if the email is not in a correct format (if the email is not valid)
if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    $this->addError('email', 'email must be a valid email');
}
child class without constructor inherits BaseClass's constructor
If there is no constructor in child class, then parent class constructor will be called (when appropriate parameters are passed) 
class BaseClass {
    function __construct() {
        print "In BaseClass constructor\n";
    }
}

class SubClass extends BaseClass {
    function __construct() {
        parent::__construct();
        print "In SubClass constructor\n";
    }
}

class OtherSubClass extends BaseClass {
    // inherits BaseClass's constructor
}

// In BaseClass constructor
$obj = new BaseClass();

// In BaseClass constructor
// In SubClass constructor
$obj = new SubClass();

// In BaseClass constructor
$obj = new OtherSubClass();
final method cannot be overridden in child classes
This example demonstrates that a final method cannot be overridden in a child class
class myClass {
    final function myFunction() {
        echo "Parent";
    }
}
// ERROR because a final method cannot be overridden in child classes.
class myClass2 extends myClass {
    function myFunction() {
        echo "Child";
    }
}
The above code will generate the following error:
Fatal error: Cannot override final method myClass::myFunction() in /home/user/scripts/code.php on line 9
self keyword
The 
self
keyword is needed to access a static property from a static method in a class definition
class myClass {
    static $myProperty = 42;
    static function myMethod() {
        echo self::$myProperty;
    }
}

myClass::myMethod();
scope resolution operator
A static property or method is accessed by using the scope resolution operator :: between the class name and the property/method name
class myClass {
   static $myStaticProperty = 42;
}

echo myClass::$myStaticProperty;
final class cannot be inherited
Child class can not extend final class
final class A {
    final public static function who() {
        echo __CLASS__;
    }
}

class B extends A {
    
}

B::who();
Fatal error: 
Class B may not inherit from final class (A)
Using $this in static methods CODE
It is not possible to use 
$this
inside static methods
class Greeting {
    public $nonStatic = 'some value';
    
    public static function hello() {
        return $this->nonStatic;
    }
}

// echo Greeting::hello();

$obj1 = new Greeting();
echo $obj1->hello();
After executing the above code, following error will be generated:
Fatal error:  Uncaught Error: Using $this when not in object context
we can use static properties inside object methods (non-static methods)
We can use static properties inside non-static methods but we can't use 
$this
inside static methods
class Circle {
    public static $pi = 3.1415926535;
    public $radius;
    
    public function __construct($radius) {
        $this->radius = $radius;
    }
    
    public function getArea() {
        return pow($this->radius, 2) * self::$pi;
    }
    
    public function getPerimeter() {
        return $this->radius * 2 * self::$pi;
    }
}

$circle1 = new Circle(5);
echo "\nArea: " . $circle1->getArea();
echo "\nPerimeter: " . $circle1->getPerimeter();
static variable in a function
There can be a static local variable in a function that is declared only once (on the first execution of a function) and can store its value between function calls
function foo()
{
   static $numOfCalls = 0;
   
   $numOfCalls++;
   echo "this function has been executed " . $numOfCalls . " times" . PHP_EOL;
}

foo();
foo();
foo();
After executing the ebove code, 
$numOfCalls
static variable gets incremented and the following text is printed:
this function has been executed 1 times
this function has been executed 2 times
this function has been executed 3 times
differences and similarities between static and constant
Differences: 
constant can not be changed
static variable can not be accessed with the class object
dollar sign is needed to access static variable inside and outside of the class
constant name can not be a keyword, e.g. class (BUT: function, self, parent, static)
Similarities : 
both of them are class members
both of them are accessible with "self" keyword inside the class
Results: 133