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";
    }
}

Classes defined as abstract cannot be instantiated

Any class that contains at least one abstract method must also be abstract

Methods defined as abstract simply declare the method's signature - they cannot define the implementation