What is the difference between abstract class and interface in PHP?

Both abstract class and interface are used to force child classes to follow a structure. But they are used for different purposes.

An abstract class is a partly built class. It can have normal methods with code and abstract methods without code. An interface is only a contract: it lists the methods that a class must have.

Feature	Abstract class	Interface
Keyword	abstract class and extends	interface and implements
Method with code	Yes	No, only method names
Properties	Yes	No (only constants)
Inheritance	A class can extend only one	A class can implement many
Method visibility	public, protected or private	public only
Best used for	Sharing common code between related classes	Giving same rules to unrelated classes

<?php
abstract class Shape {
    abstract public function area();

    public function describe() {
        return "Area is " . $this->area();
    }
}

class Square extends Shape {
    private $side;

    public function __construct($side) {
        $this->side = $side;
    }

    public function area() {
        return $this->side * $this->side;
    }
}

$sq = new Square(4);
echo $sq->describe();
?>

Output:

You can not create an object of an abstract class or an interface directly. Only the child class that completes all methods can be used to create objects.

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