An interface is like a class with nothing but abstract methods. All methods of an interface must be public. It is also possible to declare a constructor in an interface. It’s possible for interface to have constants(can not be overridden by a class/interface that inherits them). interface keyword is used to create an interface in php.

• Interface don’t have instance variables.
• All methods of an interface is abstract.
• All methods of an interface are automatically (by default) public.
• We can not use the private and protected specifiers when declaring member of an interface.
• We can not create object of interface.
• More than one interface can be implemented in a single class.
• A class implements an interface using implements keyword.
• If a class is implementing an interface it has to define all the methods given in that interface.
• If a class does not implement all the methods declared in the interface, the class must be declared abstract.
• The method signature for the method in the class must match the method signature as it appears in the interface.
• Any class can use an interface’s constants from the name of the interface like Test::roll.
• Classes that implement an interface can treat the constants as they were inherited.
• An interface can extend (inherit) an interface.
• One interface can inherit another interface using extends keywords.
• An Interface can not extends classes.

Defining Interface:

Syntax: - 
interface interface_name 
{
	const properties;
	Method;
} 

Ex: - 
interface Father  
{
	const a;
	public function disp( );
} 

Extending Interface:

• An interface can extend (inherit) an interface.
• One interface can inherit another interface using extends keywords.
• An Interface can not extends classes.

One Interface extending one Interface:

interface interface_name1
{
	const properties;
	Methods;
}

interface interface_name extends interface_name1
{
	const properties;
	Method;
}

One interface can extend more than one interface:

interface Father
{

}

interface Mother
{

}

interface Son extends Father, Mother
{

}

Implementing Class:

• More than one interface can be implemented in a single class.
• A class implements an interface using implements keyword.
• If a class is implementing an interface it has to define all the methods given in that interface.
• If a class does not implement all the methods declared in the interface, the class must be declared abstract.
• The method signature for the method in the class must match the method signature as it appears in the interface.
• Any class can use an interface’s constants from the name of the interface like Test::roll.
• Classes that implement an interface can treat the constants as they were inherited.

one interface can be implemented in a single class:

interface interface_name
{
	const properties;
	Methods;
}

class class_name implements interface_name
{
	Properties;
	Methods;
}

More than one interface can be implemented in a single class:

{

}

interface Mother
{

}

class Son implements Father, Mother
{

}

Abstract Class:

A class that is declared with abstract keyword, is known as abstract class in PHP. It can have abstract and non-abstract methods. It needs to be extended and its method implemented. objects of an abstract class cannot be created.

abstract class Test
{

}

Abstract Method:

A method that is declared as abstract and does not have implementation is known as abstract method

abstract function disp( ); //no body and abstract

abstract class Father 
{
	abstract function disp ( );
}

Class Son extends Father
{
	public function disp ( )
	{
		echo “Abstract defined”;
	}
}

Rules:

• We cannot use abstract classes to instantiate objects directly.
• objects of an abstract class cannot be created.
• The abstract methods of an abstract class must be defined in its subclass.
• If there is any abstract method in a class, that class must be abstract.
• A class can be abstract without having abstract method.
• It is not necessary to declare all methods abstract in a abstract class.
• We cannot declare abstract constructors or abstract static methods.
• If you are extending any abstract class that have abstract method, you must either provide the implementation of the method or make this class abstract.

Difference between interface and abstract class:

• An abstract class can have only abstract methods or only non-abstract methods or both, but All methods of an interface are abstract by default.
• An abstract class can declare properties/methods with access specifier, but interface can declare only constant properties and methods are by default abstract and public.
• A class can inherit only one abstract class and multiple inheritance is not possible for abstract class but A class can implement more than one interface and can achieve multiple inheritance.
• If a class contains even a single abstract method that class must be declared as abstract.
• In an abstract class, you can defined as well as It’s body methods but in the interface you can only define your methods.