AssertionTest.php

<?php

class CallbackCaller
{
	public function on($callback)
	{
		// whoops doesn't fire, perhaps due to code error
	}
}

class AssertionTest extends \PHPUnit_Framework_TestCase
{
	public function testAssertionInCallback()
	{
		$class = new CallbackCaller();
		$class->on(function() {
			$this->assertTrue(false);
		});

		$this->assertTrue(true);
	}
}

<?php
class CallbackCaller
{
	public function on($callback)
	{
            $this->_callback = $callback;
	}

        public function run() {
            // whoops doesn't fire, perhaps due to code error
            $callback = $this->_callback;
            return $callback();
        }
}
class AssertionTest extends \PHPUnit_Framework_TestCase
{
	public function testAssertionInCallback()
	{
                $localVar = false;

		$class = new CallbackCaller();
		$class->on(function() use ($localVar) {
			$localVar = true;
		});

		// run code that should run the callback
		$class->run();

		$this->assertTrue($localVar);
	}
}

Right! This is a possible solution to the initial poorly written test.

Not sure any better ways, always done callback checks like that

your test should just be checking its called, nothing else. Should be a mocked callback that only sets the variable.

Another test should test what the callback actually does IRL