Intermittent PHP Abstract Class Error

How to declare Abstract Class?

Class must also use abstract keyword

<?php
  abstract  class Book
{
     abstract public function ClassName();
}

 class firstBook extends Book{
    public function ClassName()
    {
        echo "Called form class firstBook";
    }

}

 class secondBook extends Book{
    public function ClassName() 
    {
        echo "Called from class secondBook";
    }
}

Fatal error: Class CI_Session_files_driver contains 1 abstract method

Please restart apache server if not working then download CodeIgniter again and replace your system/ directory with a fresh one.

What is the meaning of abstract in interface

Interface is an empty class which contains only the declaration of methods.So any
class which implements this interface must contain the declared functions in it. So,
interface is nothing but a strict ruling, which helps to extend any class and strictly
implement all methods defined in interface.

When a method is declared as abstract, it means that the subclass must override that
method. An abstract method should not contain any body where it is defined. An
abstract method can be declared as shown here:
abstract public function test();

If you read carefully above paragraph then you can understand All methods in an interface are abstract.

I think you are confusing becaue abstract keyword is showing before functions.

ArrayAccess {
    /* Methods */
    abstract public boolean offsetExists ( mixed $offset )
    abstract public mixed offsetGet ( mixed $offset )
    abstract public void offsetSet ( mixed $offset , mixed $value )
    abstract public void offsetUnset ( mixed $offset )
   }

Here abstract is showing function is abstract function.

serialized string contains a reference to a class that cannot be instantiated

Simple example:

abstract class Foo {}
unserialize('O:3:"Foo":0:{}');

gives:

Fatal error: Uncaught Error: Cannot instantiate abstract class Foo in /in/A4TCW:4 Stack trace:
#0 /in/A4TCW(4): unserialize('O:3:"Foo":0:{}')
#1 {main}   thrown in /in/A4TCW on line 4

It doesn't matter whether it's just the root object or a property thereof:

abstract class Foo {}
class Bar extends Foo {}
unserialize('O:3:"Bar":1:{s:1:"a";O:3:"Foo":0:{}}');

you get the same result:

Fatal error: Uncaught Error: Cannot instantiate abstract class Foo in /in/ZkdMk:5
Stack trace:
#0 /in/ZkdMk(5): unserialize('O:3:"Bar":1:{s:...')
#1 {main}
  thrown in /in/ZkdMk on line 5

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Unserializing something that simply extends an abstract class is not a problem, and is not what your quote is suggesting.

abstract class Foo {}
class Bar extends Foo {}
unserialize('O:3:"Bar":0:{}');

works fine.

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