Helper method to Mock a Guzzle Client in a PHPUnit test within a Laravel Application

MyUnitTest.php

<?php

namespace Tests\Unit;

use Tests\TestCase;
use GuzzleHttp\Client;
use GuzzleHttp\HandlerStack;
use GuzzleHttp\Psr7\Response;
use GuzzleHttp\Handler\MockHandler;

class MyUnitTest extends TestCase
{
    /**
     * @test
     */
    function testThatThingsHappen()
    {
        $responseString = '{"id": 10, "name": "Bob Edwards"}';
        $response = new Response(
            200,
            ['headers' => ['Authorization' => 'Bearer Foo']],
            $responseString
        );

        $this->mockClient($response);
        // $this->mockClient([$response, $secondResponse, $etc]); For Multiple Responses
        
        $request = app(Client::class)->get('endpoint');
        $response = json_decode((string) $request->getBody());

        $this->assertEquals(
            'Bob Edwards',
            $response->name
        );
    }

    protected function mockClient($responses)
    {
        // fluff and sanitize parameter
        if (! is_array($responses)) {
            $responses = [$responses];
        }

        foreach ($responses as $response) {
            if (!$response instanceOf Response) {
                throw new \Exception('Passed Responses must be an instance of \GuzzleHttp\Psr7\Response.');
            }
        }

        // mock everything
        $mockHandler = new MockHandler($responses);
        $handlerStack = HandlerStack::create($mockHandler);
        $client = new Client(['handler' => $handlerStack]);

        app()->instance(Client::class, $client);
    }
}