Mocks are dangerous

mocks.php

<?php

// Our test
$songs = M::mock('SongCollection');
$songs->shouldReceive('sum')
    ->with('length')
    ->andReturn(500);

$album->songs = $songs;

$this->assertEquals(500, $album->getTotalLength());

// This implementation passes
public function getTotalLength()
{
    return $this->songs->sum('length');
}

// This implementation fails even though it gives the same result with a real song collection
public function getTotalLength()
{
    $result = 0;
    foreach ($this->songs as $song) {
        $result += $song->length;
    }
    return $result;
}

// This implementation also fails even though it would give the same result with a real song collection
public function getTotalLength()
{
    return $this->songs->reduce(function($total, $song) {
        return $total + $song->length;
    }, 0);
}