Static Methods and Properties In PHP

In my last tutorial, What Object-Oriented Programming Is NOT, I mentioned that over-using or mis-using the static keyword is one way developers get confused with object-oriented programming. That, often, they simply take their procedural code, dump into a class, make everything static and away we go.

That said, there CAN be legitimate uses for fully static classes.

I like the answer Pascal MARTIN gave on StackOverflow:

The distinction I make is this:

  • Use an instantiated class for actual objects in your applications (post, category, user, etc)
  • Use a static class for “libraries of code” that have useful functions

As Pascal said, that’s probably over-simplified, but it helps me keep my head straight. Of course, the question becomes… how do we write a static class.

The first thing to keep in mind is “once you go static, you can’t go back”… in a way. More specifically, you can’t use object properties or methods in static methods. So, this won’t work:

<?php
class Library {
    public $var = 'Hey';
    
    public static function do_stuff() {
        echo $this->var;
    }
}

Library::do_stuff(); // Triggers: Fatal error: Using $this when not in object context

That’s because $this references and instantiated object. But, we bypass that when using static methods. So, $this essentially doesn’t “exist”. But, you might think you could do this:

<?php
class Library {
    public $var = 'Hey';
    
    public static function do_stuff() {
        echo Library::$var;
    }
}

Library::do_stuff(); // Triggers: Fatal error: Access to undeclared static property: Library::$var

So, once you declare a method static… basically everything it touches has to be static, too. Thus, this is what works:

<?php
class Library {
    public static $var = 'Hey';
    
    public static function do_stuff() {
        echo Library::$var;
    }
}

Library::do_stuff(); // Outputs "Hey"

One way you might use this, then, is something like this:

<?php
class Library {
    public static $date_format = 'F jS, Y';
    
    public static function format_date($unix_timestamp) {
        return date(self::$date_format, $unix_timestamp);
    }
}

echo Library::format_date(time()); // Outputs formatted date

The format_date() method is one that could be used in various places throughout your application and wouldn’t need constantly re-instantiated to use. And, of course, this Library class could contain various, similar methods that have data-agnostic uses. A kind of “catch-all” for miscellaneous functions.

And, of course, as you’ve seen the way to make a property or method static is to simply add the “static” keyword to the declaration, like so:

<?php
class Library {
    public static $var = 'Hey';
    
    public static function do_stuff() {
        // do stuff
    }
}

So, there you go. Now, if you want to keep going and learn how to build professional PHP applications using object-oriented programming, then check out my full object-oriented programming course here (also available on Udemy here).

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