Within a class, you can have dynamic or object variables and methods, where the code and the values associated with it are applied to individual objects. They're far and away the majority of your variables / methods - in yesterday's public transport service exercise, each train or bus had a potentially different start time, for example.

But just occasionally, you need a static or class variable or method, which applies to the class as a whole.

In PHP, you can define a static variable within your class using the static keyword (and you should initialise it as you do so):

  static $id_service = 0;

You can then reference that variable within both static and dynamic methodsusing the notation self::...:

  self::$id_service++;

You can define static or class functions too, but they can only access static variables - they do NOT have any $this-> access:

  static function next_id() {
    return self::$id_service + 1;
  }

Within the calling code, you access the static method using the notation class::method:

  print ("The next service will have id ". service::next_id() ."\n");

There is a complete example [here] from which the above lines of code have been "Cherry Picked"

When should you use static members? Very rarely indeed!! There's a great temptation to use them to hold a count of the number of objects you have of a type, but that approach can fall over on you if you want to create a number of different groups - you should hold them in arrays and count the arrays if that's what you need to do. My example for this post uses a static variable to assign a unique ascending ID number to each object of a type; that's subtley different, and a good use of statics.

(written 2010-03-16)

 H108 - Objects in PHP
  [3609] How do classes relate to each other? Associated Classes - (2012-02-12)
  [3608] Design Patterns - what are they? Why use them? - (2012-02-12)
  [3607] Designing your application - using UML techniques - (2012-02-11)
  [3211] Computer Graphics in PHP - World (incoming data) to Pixel (screen) conversion - (2011-03-24)
  [3210] Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON - (2011-03-22)
  [3142] Private and Public - and things between - (2011-01-22)
  [2922] Getting the OO design write - with PHP a example - (2010-08-14)
  [2921] Does copying a variable duplicate the contents? - (2010-08-14)
  [2774] PHP - Object Oriented Design in use - (2010-05-21)
  [2741] What is a factory? - (2010-04-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2641] Object Oriented Programming in PHP - (2010-02-19)
  [2632] Shipping a test harness with your class in PHP - (2010-02-12)
  [2435] Serialization - storing and reloading objects - (2009-10-04)
  [2434] Abstract classes, Interfaces, PHP and Java - (2009-10-03)
  [2380] Object Oriented programming - a practical design example - (2009-08-27)
  [2172] PHP4 v PHP5 - Object Model Difference - (2009-05-11)
  [2171] Cleaning up redundant objects - (2009-05-11)
  [2169] When should I use OO techniques? - (2009-05-11)
  [2160] PHP - getclass v instanceof - (2009-05-07)
  [1925] Introduction to Object Oriented Programming - (2008-12-06)
  [1820] Sorting objects in PHP - (2008-10-04)
  [1819] Calling base class constructors - (2008-10-03)
  [1682] Accounts in PHP - an OO demo - (2008-06-19)
  [1535] OO PHP demonstration - comparing objects and more - (2008-02-08)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1153] Object Oriented Model - a summary of changes from PHP4 to PHP5 - (2007-04-18)
  [1027] Cue the music, I'm happy. - (2007-01-09)
  [836] Build on what you already have with OO - (2006-08-17)
  [720] Planning a hotel refurb - an example of a Gant chart in PHP - (2006-05-14)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [485] North, Norther and Northest - PHP 5 Objects - (2005-11-04)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [343] Should I use structured or object oriented? - (2005-06-10)
  [205] PHP5 lets you say no - (2005-02-07)
  [124] PHP v Java - (2004-11-20)
  [67] Object Oriented Programming in PHP - (2004-09-29)

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Forward to Tcl - a great engineering language

Exception handling in PHP
Car Parking in Melksham
Adding extensions to PHP Open Source applications - callbacks
Tcl - a great engineering language
Static class members in PHP - a documented example
How to build a test harness into your PHP
Reaching out to the community - a good case for support
Blowing out the winter cobwebs
Changing Times
Redirecting to your main domain for correct security keys