The document discusses key concepts in object-oriented programming (OOP) in PHP including: - Access specifiers like public, private, and protected that control visibility of properties and methods. - The $this variable that references the current class instance from within the class. - Inheritance where a child class inherits properties and methods from a parent class. PHP only supports single inheritance. - Polymorphism where methods can take different forms. - Overriding vs overloading methods. PHP supports method overriding but not overloading. - Static properties and methods that are accessed via the class rather than object instances.

1. OOPs in PHP
http://rajvishwakarma15.blogspot.in/2014/01/oops-
abstraction-encapsulation.html
http://www.phpzag.com/overloading-and-overriding-
in-php/

2. Access Specifiers
OOps

3. $this
 $this is a special self-referencing
variable that is used to access
properties and methods from within
the class.

4. Visibility in PHP Classes
 Available Visibility in PHP Classes
 Access specifiers specify the level of access .
 There are 3 type of visibility available in php for
controlling your property or method.
 Public: Public method or variable can be accessible
from anywher. I mean from inside the class, out side the
class and in child(will dicuss in next chapter) class also.
 Private: Method or property with private visibility can
only be accessible inside the class. You can not access
private method or variable from outside of your class.
 Protected: Method or variable with protected visibility
can only be access in the derived class. Or in other
word in child class. Protected will be used in the
process of inheritance.

5. INHERITANCE

6. inheritance has three types, single, multiple and multi level inheritance. But, PHP
supports only single inheritance, where, only one class can be derived from single
parent class
PHP OOP does not allow multiple inheritance, it allow only multilevel
inheritance.
In simple word, subclass can not extend more than one super class.
But PHP allow hierarchical inheritance, Hierarchical inheritance means child
can get property of their parent and parent can get property of grand parent,
We can inherit parent class properties and function if and only if they have
public or protected access modifiers.

7. class grandParent
{
//Body of grand parent class
}
class parent extends grandParent
{
//Body Of parent class
}
class child extends parent
{
//Body of child class
}
, inheritance has three types, single, multiple and multi level inheritance.
But, PHP supports only single inheritance, where, only one class can be
derived from single parent class. Even though,
PHP is not supporting any other inheritance type except single
inheritance, we can simulate multiple inheritance by using PHP
interfaces.

8. Polymorphism
Polymorphism is derived from 2 greek
words: poly and morphs. The word "poly"
means many and "morphs" means forms. So
polymorphism means many forms.
Polymorphism means one name many forms. One
function behaves different forms.

9. Overloading Same method name with different signature,
since PHP doesn’t support method overloading concept
=> Overriding When same methods defined in parents and
child class with same signature i.e know as method overriding

10. STATIC
STATIC DATA MEMBERS
AND METHODS

11. To declare a static property using static keyword.
Static properties of class is a property which is directly accessible from class
with the help of :: (scope resolution operator).
class test
{
public static $a;//Static variable
}
test::$a = 5;
echo test::$a;
//-class test
{
static function abc($param1 , $param2)
{
echo "$param1 , $param2";
}
}
test::abc("ankur" , "techflirt");

12. $this self
Represents an instance of the class
or object
Represents a class
Always begin with dollar ($) sign Never begin with dollar($) sign
Is followed by the -> operator Is followed by the :: operator
The property name after the ->
operator does not take dollar ($)
sign, e.g., $this->property.
The property name after the ::
operator always take the dollar ($)
sign.

13. Non-static
variable
Static variable
These variable
should not be
preceded by any
static keyword
Example:
class A
{
int a;
}
These variables
are preceded by
static
keyword.Example
class A
{
static int b;
}