PHP Book - The Trait

PriceUtilities.php

<?php 

/**
 * 
 * Traits
 * 
 * 1. PHP does not allow for multiple inheritance
 * 2. A class can implement many interfaces, but interfaces only provide a template. 
 * 3. Traits allow us to share implementation across class hierarchies. 
 * 
 * Characteristics: 
 * 
 * 1. Like an abstract class, a trait cannot itself be instantiated
 * but can be incorporated into classes. 
 * 
 * 2. Any methods defined in a trait become available as part of any class that uses it. 
 * 
 * 3. A trait changes the structure of a class, but doesn't change its type - 
 * it's like an include for classes! This can prevent duplication
 * 
 * 4. Like interfaces (and inline with the include comparison), you can use as many as 
 * necessary within a class. 
 * 
 * 5. You can also use abstract classes, static classes and even access host 
 * class proprties (though this is poor design from within a trait
 * 
 * 6. You can even change a method's access level from within the host class. 
 * 
 * 
 * Reference: PHP Objects, Patterns and Practice, ed 4. pp. 45-49, 52-53
 **/
 
 trait PriceUtilities {
   private $taxRate = 0.0750;
   
   function calculateTax($price) {
     return $price * (1 + $taxRate);
   }
   
   ...
 }

///

/**
 * Both of these classes have access to the calculateTax method
 **/

class Product {
  use PriceUtilities;
  ...
}

class Register {
  //use PriceUtilities; 
  /** Example of changing scope of method **/
  use PriceUtilities {
   PriceUtilities::calculateTax as private;
  }
  ...
}

$r = new Register();
$p = new Product();

/** this would not work because method is now private **/
echo $r->calculateTax(16.55);
echo $p->calculateTax(15.55);

/**
 * Combining Traits with Interfaces.
 * 
 * Traits don't change the type of the class to which they are applied.
 * So when you apply the IdentityTrait to multiple classes, they won't share 
 * a type. 
 * 
 * So if an interface requires a method, you can use a trait to include this method 
 * rather than putting into the class directly. -- Allows type hinting to continue
 **/