In this tutorial ,I will explain about abstract class in PHP, how to declare and use of an abstract class.

 Abstract Class:
It may contain one or more abstract methods.abstract classes may not be instantiated.The child classes which inherits the property of abstract base class, must define all the methods declared as abstract.
Any class that contains at least one abstract method must also be abstract.
if the abstract method is defined as protected, the function implementation must be defined as either protected or public, but not private.

Example:

<?php
abstract class AbstractClass
{
    abstract protected function getValue();
    abstract protected function setValue($val);

    public function Display() {
        print $this->getValue() . "\n";
    }
}

class ChildClass1 extends AbstractClass
{
    protected function getValue() {
        return "ChildClass1";
    }

    public function setValue($val) {
        return "ChildClass{$val}";
    }
}

class ChildClass2 extends AbstractClass
{
    public function getValue() {
        return "ChildClass2";
    }

    public function setValue($val) {
        return "ChildClass{$val}";
    }
}

$class1 = new ChildClass1;
$class1->Display();
echo "<br>";
echo $class1->setValue('1') ."\n";
echo "<br>";
$class2 = new ChildClass2;
$class2->Display();
echo "<br>";
echo $class2->setValue('2') ."\n";
?>