PHPUnit error on mock expecting interface

SampleProduct.php

class SampleProduct
{
    /* implementation */

    public function setBougthBy(UserInterface $user)
    {
        $this->bougthBy = $user;
    }
}

SampleProductTest.php

class SampleProductTest
{
    /* other tests */

    public function testSetBougthByWithValidUserShouldSuccess()
    {
        $stubUser = $this->getMock('SampleUser');

        $product = new SampleProduct();
        
        /**
         * PHPUnit_Framework_Error
         * Expected UserInterface, got Mock_MockObject
         */
        $product->setBougthBy($stubUser);
    }
}

SampleUser.php

<?php

class SampleUser implements UserInterface
{
    /* Implementation */
}

UserInterface.php

<?php

interface UserInterface
{
    /* methods */
}

Use the getMockBuilder method:

//...

$stubUser = $this->getMockBuilder('UserInterface')
                 ->setMethods(array('someMethod`))
                 ->getMock();

$product = new SampleProduct();

$product->setBougthBy($stubUser);
//...

;)