Example of PHP shell helper injection and test

index.php

<?php

// If this function were a class method, the injection could happen in the constructor instead
function printMessageAndExit(string $message, ?ShellHelper $shell): void {
	$shell = $shell ?? new DefaultShell();
	$shell->echo($message);
	$shell->exit();
}

interface ShellHelper {
	public function echo(string $message): void;
	public function exit(): void;
}

class DefaultShell implements ShellHelper {
	public function echo(string $message): void {
		echo $message;
	}

	public function exit(): void {
		exit();
	}
}

test.php

<?php

class MyTest extends \PHPUnit\Framework\TestCase {
	public function testPrintMessage() {
		$helper = new class implements ShellHelper {
			public $echoedMessage;
			public $didExit;
			
			public function echo(string $message): void {
				$this->echoedMessage = $message;
			}

			public function exit(): void {
				$this->didExit = true;
			}
		};
		
		printMessageAndExit('hello', $helper);
		$this->assertEquals('hello', $helper->echoedMessage);
		$this->assertTrue($helper->didExit);
	}
}