Update php.html.markdown

a few sentences to constants

php.html.markdown

@@ -99,6 +99,21 @@ END;
 echo 'This string ' . 'is concatenated';
 
 
+/********************************
+ * Constants
+ */
+ 
+// A constant is defined by using define()
+// and can never be changed during runtime!
+
+// a valid constant name starts with a letter or underscore,
+// followed by any number of letters, numbers, or underscores.
+define("FOO",     "something");
+
+// access to a constant is possible by direct using the choosen name
+echo 'This outputs '.FOO;
+
+
 /********************************
  * Arrays
  */