How to mock a final class and private method

TestFinalClass.php

<?php 
namespace UnitTest\App;

final class FinalClass
{
    public function __construct()
    {

    }

    /**
     * int $number
     * 
     * @retun int
     */
    private function IamPrivate(int $number): int
    {
        return ($number!=2) ? $number : 0;
    }
    
    /**
     * int $number
     * 
     * @retun int
     */
    public function getNumber($num): int
    {
        return $num;
    }

}
?>

//----------------------------------------------------------------------------------

<?php

use UnitTest\App\FinalClass;
use PHPUnit\Framework\TestCase;
use DG\BypassFinals;

final class FinalClassTest extends TestCase
{

    protected $finalClass;
    private $mock;
    private $reflector;

    protected function setUp(): void
    {
        BypassFinals::enable(); //only required for mocking but not required in reflection
        $this->finalClass = new FinalClass();

    }

    /**
     * How to mock a final class
     * 
     * @test
     * @return void
     */
    public function getNumber(): void
    {
        $this->mock = $this->createMock(FinalClass::class);
        $this->mock->method('getNumber')
            ->with(3)
            ->willReturn(3);
        $result = $this->mock->getNumber(3);
        $this->assertEquals(3, $result);
       
    }
    /**
     * How to access the private method using reflection class
     * 
     * @test
     * @return void
     */
    public function IamPrivate(): void
    {
        $number = 2;
        $method = $this->getPrivateMethod( FinalClass::class, 'IamPrivate' );;
        $result = $method->invokeArgs( $this->finalClass, array( $number ) );
        $this->assertEquals( 0, $result );

    }

    
	/**
 	 * getPrivateMethod
 	 *
 	 * @author	Gopal Dasbairagya <gopal.dasbairagya@tcs.com>
 	 * @param 	string $className
 	 * @param 	string $methodName
 	 * @return	ReflectionMethod
 	 */
	public function getPrivateMethod( $className, $methodName ) {
		$reflector = new ReflectionClass( $className );
		$method = $reflector->getMethod( $methodName );
		$method->setAccessible( true );

		return $method;
	}
}

?>

You need to run composer require dg/bypass-finals --dev if you want to only mock a final class. For reflection it is not required.