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copied!<p>IMHO, you should use Polymorphism.<br> <a href="http://www.youtube.com/watch?v=4F72VULWFvc" rel="noreferrer">This Video</a> may help you understanding this principle</p> <p>Here's my way of thinking.</p> <p>First, define an interface for any operations you'd need</p> <pre><code>interface OperationInterface { public function evaluate(array $operands = array()); } </code></pre> <p>Then, create the calculator holder</p> <pre><code>class Calculator { protected $operands = array(); public function setOperands(array $operands = array()) { $this->operands = $operands; } public function addOperand($operand) { $this->operands[] = $operand; } /** * You need any operation that implement the given interface */ public function setOperation(OperationInterface $operation) { $this->operation = $operation; } public function process() { return $this->operation->evaluate($this->operands); } } </code></pre> <p>Then you can define an operation, for example, addition</p> <pre><code>class Addition implements OperationInterface { public function evaluate(array $operands = array()) { return array_sum($operands); } } </code></pre> <p>And you would use it like :</p> <pre><code>$calculator = new Calculator; $calculator->setOperands(array(4,2)); $calculator->setOperation(new Addition); echo $calculator->process(); // 6 </code></pre> <p>With that point, if you want to add any new behaviour, or modify an existing one, just create or edit a class.</p> <p>For example, say you want a Modulus operation</p> <pre><code>class Modulus implements OperationInterface { public function evaluate(array $operands = array()) { $equals = array_shift($operands); foreach ($operands as $value) { $equals = $equals % $value; } return $equals; } } </code></pre> <p>Then, </p> <pre><code>$calculator = new Calculator; $calculator->setOperands(array(4,2)); $calculator->setOperation(new Addition); // 4 + 2 echo $calculator->process(); // 6 $calculator->setOperation(new Modulus); // 4 % 2 echo $calculator->process(); // 0 $calculator->setOperands(array(55, 10)); // 55 % 10 echo $calculator->process(); // 5 </code></pre> <hr> <h3>This solution allows your code to be a third-party library</h3> <p>If you plan to reuse this code or give it away as a library, the user wouldn't by any case <em>modify your source code</em>.<br> But what if he wants a <code>Substraction</code> or a <code>BackwardSubstraction</code> method which are not defined ? </p> <p>He just has to create his very own <code>Substraction</code> class in his project, which implements <code>OperationInterface</code> in order to work with your library.</p> <h3>It's easier to read</h3> <p>When looking in the project architecture it's easier to see a folder like this</p> <pre><code>- app/ - lib/ - Calculator/ - Operation/ - Addition.php - Modulus.php - Substraction.php - OperationInterface.php - Calculator.php </code></pre> <p>And immediatly know which file contains the desired behaviour.</p>

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