Create your custom PHPUnit Constraint

Code
<?php

/**
 * Constraint, that checks if a class has one or more interfaces
 */
class HasInterface extends PHPUnit_Framework_Constraint
{
    protected function 
matches($other)
    {
        if (!
class_exists($other)) {
            return 
false;
        }
        
        return 
count(class_implements($other)) !== 0;
    }

    public function 
toString()
    {
        return 
'has one or more interfaces';
    }
}

class 
ConstraintTest extends PHPUnit_Framework_TestCase
{
    
/**
     * fails, because 6 is no class
     */
    
public function testIsNoClass()
    {
        
$this->assertThat(6, new HasInterface());
    }
    
    
/**
     * fails, because SoapClient has no interface
     */
    
public function testHasNoInterface()
    {
        
$this->assertThat('SoapClient', new HasInterface());
    }
    
    
/**
     * works, because ArrayIterator has one or more interfaces
     */
    
public function testHasInterface()
    {
        
$this->assertThat('ArrayIterator', new HasInterface());
    }
}
Result
Time: 195 ms, Memory: 2.00MB

There were 2 failures:

1) ConstraintTest::testIsNoClass
Failed asserting that 6 has one or more interfaces.

ConstraintTest.php:30

2) ConstraintTest::testHasNoInterface
Failed asserting that 'SoapClient' has one or more interfaces.

ConstraintTest.php:38

FAILURES!
Tests: 3, Assertions: 0, Failures: 2.