Can an abstract class contain a private method and why?

Yes, it can.

An abstract class can contain private methods and properties. Private members of a class are accessible only within the class itself. Even classes that inherit from the abstract class do not have direct access to its private members.

This is useful for encapsulating internal logic or auxiliary operations that are used only within the abstract class itself and are not intended to be implemented or modified in derived classes.

Example:

<?php

abstract class AbstractClass
{
    private function internalHelperMethod(): void
    {
        // Auxiliary logic used only inside AbstractClass
        echo "Internal helper function called.\n";
    }

    protected function executeInternalHelper(): void
    {
        // Call private method from a protected method accessible to descendants
        $this->internalHelperMethod();
    }

    abstract public function doSomething(): void;
}

class ConcreteClass extends AbstractClass
{
    public function doSomething(): void
    {
        echo "Implementation of doSomething.\n";
        // Cannot directly call $this->internalHelperMethod(); from ConcreteClass
        $this->executeInternalHelper(); // But can call through the protected method
    }
}

// $abstract = new AbstractClass(); // Error: Cannot instantiate an abstract class

$concrete = new ConcreteClass();
$concrete->doSomething();

?>