Update README.md

Author: petrknap

README.md

@@ -8,6 +8,36 @@ Enumerated type for PHP by [Petr Knap].
 > In computer programming, an **enumerated type** (also called **enumeration** or **enum**, or **factor** in the R programming language, and a categorical variable in statistics) is a data type consisting of a set of named values called **elements**, **members**, **enumeral**, or **enumerators** of the type. The enumerator names are usually identifiers that behave as constants in the language. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value. In other words, an *enumerated type has values that are different from each other*, and that can be compared and assigned, but which are not specified by the programmer as having any particular concrete representation in the computer's memory; compilers and interpreters can represent them arbitrarily.
 -- [Enumerated type - Wikipedia, The Free Encyclopedia]
 
+## Why use enum?
+
+Because it is safer then using class constants.
+
+```php
+class MyBoolen
+{
+    const MY_TRUE = 1;
+    const MY_FALSE = 2;
+}
+
+function IsTrue($myBoolean)
+{
+    switch($myBoolean) {
+        case MyBoolen::MY_TRUE:
+            return true;
+        case MyBoolen::MY_FALSE:
+            return false;
+    }
+}
+
+IsTrue(MyBoolen::MY_TRUE);  // returns true - OK
+IsTrue(MyBoolen::MY_FALSE); // returns false - OK
+IsTrue(1);                  // returns true - OK
+IsTrue(2);                  // returns false - scary, but ok
+IsTrue(true);               // returns true - OK
+IsTrue(false);              // returns null - WTF?
+```
+
+
 ## Usage of php-enum
 
 ### Enum declaration