PHP Fatal error: Interface not found, using factory pattern with namespaces

File 1 - /Users/jitendraojha/www/DesignPatternsWithPhpLanguage/FactoryPattern/FactoryClassPattern/UserInterface.php

<?php

namespace DesignPatternsWithPhpLanguage\FactoryPattern\FactoryClassPattern;

interface UserInterface
{

    function setFirstName($firstName);
    function getFirstName();
}

?>

File 2 - /Users/jitendraojha/www/DesignPatternsWithPhpLanguage/FactoryPattern/FactoryClassPattern/User.php

<?php

namespace DesignPatternsWithPhpLanguage\FactoryPattern\FactoryClassPattern;


class User implements UserInterface
{

    private $firstName = null;

    public function __construct($params) {   }

    public function setFirstName($firstName)
    {

        $this->firstName = $firstName;
    }

    public function getFirstName()
    {

        return $this->firstName;
    }
}

?>

Problem

php FactoryPattern/FactoryClassPattern/UserInterface.php - Runs fine.

php FactoryPattern/FactoryClassPattern/User.php - gives following errors: PHP Fatal error: Interface 'DesignPatternsWithPhpLanguage\FactoryPattern\FactoryClassPattern\UserInterface' not found in /Users/jitendraojha/www/DesignPatternsWithPhpLanguage/FactoryPattern/FactoryClassPattern/User.php on line 7

I had use UserInterface; added in File 2 with no solution.