Why does PHPUnit require an intermediate variable when mocking/stubbing?

Question

I am doing some unit testing with PHP. I got stuck on an error that the fluent interface of PHPUnit requires an intermediate variable. What is the reason for this behavior?

See MyTests::works vs. MyTests::fails:

<?php
use PHPUnit\Framework\TestCase;

interface SomeInterface {
    public function someMethod();
}

class SomeService {
    public function __construct(
        private SomeInterface $interface
    ) { }

    public function testMethod() {
        $this->interface->someMethod();
    }
}

class MyTests extends TestCase {
    // Using fluent-interface with intermediate variable works
    public function works() {
        $mock = $this->createMock(SomeInterface::class);
        $mock->expects($this->once())
                    ->method("someMethod");

        $service = new SomeService($mock);
        $service->testMethod();
    }

    // Using fluent-interface without intermediate variable fails
    public function fails() {
        // Fails: TypeError: SomeService::__construct(): Argument #1 ($interface) must be of type SomeInterface, PHPUnit\Framework\MockObject\InvocationStubberImplementation given, called in /xxx/MyTests.php on line 25
        $mock = $this->createMock(SomeInterface::class)
                    ->expects($this->once())
                    ->method("someMethod");

        $service = new SomeService($mock);
        $service->testMethod();
    }
}

?>

I already found the same question asked before (Mock class with fluent interface in Phpunit), but I am searching for the why not for the fix.

jonrsharpe · Accepted Answer · 2025-10-30 14:17:56Z

5

createMock returns a MockObject. But expects and method return an InvocationStubber.

So for the fluent interface chain

$variable = $this->createMock(...)->expects(...)->method(...)

the return types change in-flight and only the return value of the last call in the chain gets stored in the variable assignment.

By preserving the return value of createMock(...) in an extra variable one prevents the type error, since only the MockObject provides the "type magic" required.

So it's just some suprising design of the fluent interface from my point of view, not a PHP-specific behavior.

edited Oct 30 at 14:17

jonrsharpe

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answered Oct 29 at 21:42

Sven Eppler

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