Abstract Keyword

PHP has abstract classes and methods. Classes defined as abstract cannot be instantiated, and any class that contains at least one abstract method must also be abstract.

<?php
abstract class AbstractClass
{
    // Force Extending class to define this method
    abstract protected function getValue();
    abstract protected function prefixValue($prefix);

    // Common method
    public function printOut() {
        print $this->getValue() . "\n";
    }
}

?>

Documentation

See also Interfaces vs Abstract Classes in PHP, Abstract classes and methods, Testing Abstract Classes in PHP using Anonymous Classes

Related : Final Keyword