WEBVTT 00:00.200 --> 00:04.640 Okay, so now let's learn about the static properties and methods. 00:04.640 --> 00:13.070 Those would be properties and methods that actually belong to the class, not any particular instance. 00:13.160 --> 00:20.120 So I'd like to remind you the analogy that we've used in the first video about classes. 00:20.270 --> 00:29.390 So a class is Tesla model Y like a general concept when you talk about Tesla in general, if you talk 00:29.420 --> 00:37.580 about Tesla, then you don't mean my Tesla or anyone else's Tesla, you just mean the model in general. 00:37.610 --> 00:41.930 Maybe this can be BMW three series. 00:41.960 --> 00:48.200 As another example, you don't talk about your uncle's BMW, but general about the car. 00:48.200 --> 00:49.640 You get the point. 00:50.870 --> 01:01.100 And sometimes you can also have some properties and some methods that belong to this generic concept 01:01.100 --> 01:04.260 to a class, not particular instance of it. 01:04.260 --> 01:11.580 And as an example, we are going to create a class that's called math utils. 01:11.580 --> 01:22.680 And what this one will do is it is a generic, let's call it container for mathematical related utilities. 01:24.480 --> 01:35.160 So for example, I might have a static float field which is initialized using a number which we know 01:35.160 --> 01:35.910 is constant. 01:35.910 --> 01:37.650 So it won't change. 01:37.650 --> 01:42.900 And it's just handy to be able to get it everywhere. 01:43.110 --> 01:52.380 And another thing what's handy about that is we can, for example, add many mathematical methods that 01:52.380 --> 01:59.130 we often use, and then we just have this one place where we know everything is placed. 01:59.130 --> 02:02.410 And that's our math utils class. 02:02.410 --> 02:10.060 Now it's totally fine to mix static and non-static fields and methods in a class. 02:10.060 --> 02:14.170 So this is just a specific example that we are having here. 02:14.440 --> 02:21.490 Let me also add a method which can be called square. 02:21.700 --> 02:23.860 So this just accepts a number. 02:23.860 --> 02:28.630 And well this method is not really specific to any instance. 02:28.630 --> 02:33.220 It's just like a function grouped under a class. 02:33.430 --> 02:39.520 So this would be just number times number. 02:40.180 --> 02:48.700 And now let's see how can we actually call those static methods and get access to static fields. 02:50.230 --> 02:53.440 So let's vardump the results. 02:53.440 --> 02:56.410 So we've got the p number. 02:57.490 --> 03:03.440 So I get access to the p number field by using the class Last name double colon. 03:03.440 --> 03:09.110 And then I access the field by prepending the name with a variable. 03:10.520 --> 03:14.690 So this is how can I vardump the value of this p field? 03:14.690 --> 03:21.590 And to call the method the same, I use the class name double colon and I can call the square method. 03:21.590 --> 03:24.380 Let's say four four. 03:26.510 --> 03:32.510 Now finally let's run this and we've got results for both. 03:33.440 --> 03:39.260 Now let me show you another example where static methods make a lot of sense. 03:39.290 --> 03:43.070 That's for example a singleton design pattern. 03:43.100 --> 03:51.020 So in short singleton means that certain class should be created only once and you should not be able 03:51.020 --> 03:52.340 to create it again. 03:52.340 --> 04:00.770 And this is important if you have, for example, expensive resources like you need to create a database 04:00.770 --> 04:01.430 connection. 04:01.430 --> 04:07.890 And you want to make sure that only one connection is created and that it is then reused. 04:09.810 --> 04:19.020 So let's create a connection class and to apply the singleton pattern, what you can do is you will 04:19.020 --> 04:21.810 make the constructor private. 04:22.050 --> 04:24.570 So this is a known PHP technique. 04:24.570 --> 04:30.930 By making constructor private no one can actually create a class. 04:31.110 --> 04:38.220 So that's connection I do new connection and I expect an error in here. 04:38.220 --> 04:41.040 So let's run this example. 04:41.490 --> 04:44.790 And we are calling a private constructor. 04:44.790 --> 04:47.310 So it throws a fatal error. 04:47.310 --> 04:50.670 So we made this constructor private. 04:50.670 --> 04:54.090 So as I said no one can create this class explicitly. 04:54.120 --> 05:01.470 But we can give another way to do that by adding a public static method. 05:01.860 --> 05:08.610 It can be called singleton, or often it's also called factory. 05:09.750 --> 05:12.360 Missing the function keyword here. 05:12.720 --> 05:20.580 Now, since the constructor is private, it means it cannot be called from outside this class, which 05:20.580 --> 05:24.210 means the class cannot be created using the new operator. 05:24.510 --> 05:29.910 But I can still call this function from within the class. 05:30.360 --> 05:36.240 So here I can do return new connection. 05:38.790 --> 05:41.100 But this is not a singleton yet. 05:42.240 --> 05:44.760 So I would have to add a field here. 05:45.420 --> 05:51.450 So I would have to add private static instance. 05:53.730 --> 05:55.980 And initialize it with null. 05:56.010 --> 06:04.690 So by making this private and static I just make sure it's not accessible People outside of the class. 06:07.990 --> 06:09.130 And I can do. 06:09.160 --> 06:12.430 If null equals. 06:15.790 --> 06:19.330 Connection instance. 06:20.890 --> 06:23.740 So I check if the static field is null. 06:24.820 --> 06:26.410 I initialize it. 06:27.340 --> 06:29.830 So I do connection. 06:32.590 --> 06:35.140 Instance equals new connection. 06:36.580 --> 06:38.830 Let me add square brackets in here. 06:42.880 --> 06:48.010 And then I always return the connection instance. 06:50.680 --> 06:59.050 So how it works is I always check if the instance one and only instance of the connection class was 06:59.050 --> 07:00.370 created already. 07:00.430 --> 07:02.740 If it wasn't, I create it. 07:02.770 --> 07:04.820 If it was, I just return it. 07:04.850 --> 07:13.280 This makes sure that there will always be at most one instance of the connection class. 07:13.610 --> 07:21.380 Now to create this connection you run the connection singleton. 07:24.260 --> 07:30.950 Okay, let's comment this out and just run this file to make sure that there is no issue. 07:31.250 --> 07:31.580 Okay. 07:31.610 --> 07:36.620 So everything went fine and we actually created a class. 07:37.520 --> 07:39.980 Now we can also make an improvement here. 07:39.980 --> 07:50.000 So instead of directly relying on the class name inside the class we can use a static keyword. 07:50.870 --> 07:56.810 So this is professionally called late static binding. 07:56.840 --> 08:05.760 And what this does is essentially if you would extend the connection class then the code will just work 08:05.760 --> 08:15.060 for the child class as well, because here if we explicitly rely on the connection class, we will always 08:15.090 --> 08:21.600 be using all the methods and fields as they were defined in the connection class. 08:21.600 --> 08:26.880 So this really makes this class less extendable. 08:26.940 --> 08:36.030 And if you remember the lecture about inheritance, it's quite possible literally almost the essence 08:36.030 --> 08:42.870 of object oriented programming to be able to extend the classes and change the functionality. 08:42.870 --> 08:50.880 So unless you have strong reasons for it, use the static keyword, not the actual class name inside 08:50.880 --> 08:58.110 the class definition baths because outside you need to use the concrete class name. 08:58.110 --> 09:03.720 Otherwise PHP would not know which static method you actually want to call.