TechTip: PHP Classes: Just When You Thought It Was Over

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I promise, this is it, the last section of our opus on creating PHP OO classes. Finally! And believe me, no one is happier than I am. Except that now I have to decide what to write about next.

Last month, we finally got a class definition that not only was I satisfied with, but also actually worked. So what is there left to do? Not too much at this point (there really is a lot more stuff about classes that we could go into, but I'm not going there yet). But we have a few "icing" kinds of things to clean up, so let's get started.

At the end our last episode, we had the following script code all rolled up into one file.

0 <?php

1   Class name

2   {   public $firstName;

3      public $lastName;

4

5       public function __construct($firstName, $lastName)

6          {     $this->firstName = $firstName;

7                $this->lastName = $lastName;

8          }

9

10         public function printName()          

11          {   echo 'My name is ' . $this->firstName . ' ' . 12                         $this->lastName;

13         }

14    }

15

16   $firstName = 'David';

17   $lastName = 'Shirey';

18    $myName = new name($firstName, $lastName);

19    $myName -> printName();

20 ?>

What we have here is, however, actually two things, and I want to separate them.

First, the class definition, taking up lines 1–14. The class contains information on the properties (variables) that we'll be using as input (lines 2, 3), the constructor function using parms of first name and last name (lines 5–8), and a method that prints the name information (lines 10–13).

Second, on lines 16–19, we start by setting the values of the parameters we want to pass into the constructor (lines 16–17), then issue the access of the constructor method by instantiating a new class object (line 18), and finally, after the new object has been created, we access the printName method to print out the information (line 19).

The result is, if you run this file from a browser URL line, you'll get the verbiage "My Name is David Shirey" to show up on the browser page.

My question to you is, have you tried this yet? Hopefully, you will try it, because believe me, you don't learn this stuff by reading. You learn it by doing. So give it a try.

Splitting into Two Files

Of course, it really makes no sense for us to have all this logic in a single file. Typically, even though the class can't be thought of as a service program or subprocedure (but rather more like an envelope or box that contains like data and processing objects), classes are generally set up in their own files. Or at most, a group of classes will be set up in a single file. At any rate, the class is separated from the logic that will access it.

To do this, simply take lines 1–14 above, put a ?> at the end, and stick it in a file of its own. Call it what you will.

Then take lines 16–20, put a <?php on the front end, and dump it into a file of its own.

Now, one more thing needs to be added to the second file in order to pull the class definition in when it's time to run this logic, because somehow we have to tie these two together.

There are two PHP commands that can be used to pull a script into another script at run time: include and require. The difference between the two is that when you specify include and the file you're including isn't there, the calling script will continue to run. If you specify require, on the other hand, then the calling script fails.

There's also a qualifier on both commands; once and not specifying once. If you specify once, then the code is included only once, even if it's accessed multiple times in the calling script. If once is not specified, then every time the called script is needed, it will need to go get the class script and load it. The format for this is require_once or include_once.

The full syntax for the include or require is this:

require_once('/filepath.php');

Discussion of Properties in a Class

I want to talk about one more thing before we bring the curtain down on classes: data within classes. Let's go back to our example above.

Class vs. Parm Variables

On lines 2 and 3, we defined what I will call the class variables: $firstName and $lastName. We haven't given a type or picture to them. That will be done by the system when a value is assigned. The class properties are local to the class but within the class are globalthat is, every method in the class can use those properties, but the properties have no meaning outside of the class.

In our example, we used the names of these class properties as input parms on the constructor on line 5. In reality, we could have named these two parms something different because they actually are different variables. For example, Chris Ringer prefixes his parm variables with a p so they show up as $pfirstName and $plastName. In reality, whether you name these two sets of properties the same or different, they're really different data fields and can have different values simultaneously. They also have different scopes of control. The $pfirstName variables are only defined within the constructor and will not work outside of that method. The $firstName variable will work anywhere in the class. Just something to keep in mind.

Now you might be thinking, Hey, I'll just use the parm variables then and forget about the class properties. The problem is that parm variables only act in the method they are in. So with just parm properties, there's no way to move data from one method to another. Only a class-level variable can pass data through the length and breadth of the class.

$this->firstName;

The other thing we want to look at is the firstname property that shows up in the $this->firstname. Is this really a property? The answer is no, not really. Although we (I) like to take things apart into their constituent pieces, the $this->firstname must be looked at as a single term or symbol. Remember that it just means that you're referring to the value in the class of the parm $firstName. Why don't you just use $firstName? That is, instead of saying '$this->firstname = $firstName', why don't you just use $firstName and be done with it? The answer is simple. Because it doesn't work that way. You can't refer to a class variable within a subordinate method. You have to use the $this verbiage. It's like Dad always used to say: "Because, just because."

What Happens When a Class Opens and Closes

One more thing I want to mention is what happens when a class is opened or called. This is done with the "new" statement because opening a class always begins with the creation of a new object in that class. Hence, the constructor should always be a part of the class definition, and this constructor is kicked off as soon as the class opens.

After that, the class just sort of sits there. That is, it doesn't end until the script that calls it ends or we call the destructor. The destructor is the opposite of the constructor in that it closes the class and deletes all objects associated with that class that are still open. Running the destructor is important because while PHP does have garbage cleanup, it doesn't happen automatically like it does in RPG.

Also, please note that if we had coded a destructor in our example, then the new object would have been created and then immediately destroyed because our script did not stay open but rather closed very quickly. A class in a more normal PHP script, like the one we defined today by splitting the class from the class caller thingy, would be attached to a web page and would stay open for much longer (until you closed that screen).

The End…for Now

There's plenty more to learn about classes, but that's it for now. You should be able to write a class, create an object in it, assign values to that object, destroy that object, and perform methods. And for now, that's enough.

 

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