Dummy.php

<?php
namespace Temp;

use ToolkitApi\Toolkit;

class Dummy
{
    /** @var Toolkit */
    private $ibmiToolkit;

    /**
     * Dummy constructor.
     * @param Toolkit $ibmiToolkit
     */
    public function __construct(Toolkit $ibmiToolkit)
    {
        $this->ibmiToolkit = $ibmiToolkit;
    }

    public function dummyMethod()
    {
        return $this->ibmiToolkit->AddParameterPackDec('both', 9, 0, 'SRN', 'SRN', 0);
    }
}

DummyTest.php

<?php
namespace TempTest;

use PHPUnit_Framework_TestCase;
use Temp\Dummy;
use ToolkitApi\Toolkit;

class DummyTest extends PHPUnit_Framework_TestCase
{
    public function testDummyMethod()
    {
        $ibmiToolkit = $this->getMockBuilder(Toolkit::class)
            ->disableOriginalConstructor()
            ->getMock();
        $ibmiToolkit->expects(self::any())
            ->method('AddParameterPackDec')
            ->withAnyParameters()
            ->willReturn('test');
        /** @var Toolkit $ibmiToolkit */

        $dummy = new Dummy(
            $ibmiToolkit
        );

        $this->assertEquals('test', $dummy->dummyMethod());
    }
}

@chukShirley I haven't seen ->withAnyParameters() before, but instead ->with($this->any())