README.md

Interface segregation principle

To me, this principle is the second important because it is a very effective way to design loose coupled system. Plus, it doesn't have performance issues.

Unlike LSP, ISP can be resumed in one phrase as "OOP is not facebook". But there is a big prerequisite : you must have interfaces everywhere.

Objects are socially awkward, they don't want attachment to many people and they don't want to deeply know their friends. Just the bare minimum to follow the contract they need, everything else does not concern them.

A bad practice

public function getTotal(\SplFixedArray $listing)
{
    $sum = 0;
    foreach($listing as $row) {
        $sum += $row->getPrice();
    }

    return $sum;
}

Why using a SplFixedArray if we just need to traverse the array ?

A good practice

public function getTotal(\Iterator $listing)
{
    $sum = 0;
    foreach($listing as $row) {
        $sum += $row->getPrice();
    }

    return $sum;
}

Since SplFixedArray inherits from Iterator, we "segregate" its type-hint to the minimum interface.

If you have big interface, you know there is something wrong with your design.

If you can't break your interface, then you might have some problem with SRP.