Variables

According to the meaning of the word, you should suspect it is something which changes all the time. Indeed the main feature of a variable is to be able to change its value. But strictly speaking, what does it mean?

A variable is a small piece of information stored in the memory temporarily. It has a short life cycle. In PHP a variable exists during the generation process of the page. Once the page has been generated, all variables are removed from the memory because they are of no use anymore. A variable is therefore not a file stored on the hard drive but a temporary piece of informaton.
Creating a variable is up to you, that is, you create it when you think it is needed.
What you should know is that a variable is made up of 2 things:

• Its name : in order to identify your variable you must name it. For instance "age_of_visitor"
• Its value : the value is the information conveyed. It can change. For example "17 years".

In this example, I used a variable called "age_of_visitor" with the value "17". The value of a variable can be changed at will and used for any calculations. And when we need it, we call it (by its name) and it tells us what it's set to. It goes on like this: - Hey ! you age_of_visitor variable, what are you set at?
- 17.
- Thanks ! Variables may seem confusing for now but they will be mandatory for your PHP site. For instance, storing a username ("m@teo21") in a name_of_visitor variable, you will be able to use this information to display a personal welcome message like, " Hi m@teo21! welcome". You remember how to display a text in PHP, don't you? The "echo" function you learned in the previous chapter will be very useful for practice!

The first thing we are going to do is assign a value to the variable and then display what it contains.
In this example we are just going to create a variable and assign it a value just for the fun of it For example if we type:Code: PHP We would create a variable:

• Named username_of_visitor
• Which value would be Mateo21

We can notice several new elements. First the dollar sign ($): a variable's name always starts with this character. It actually tells PHP that you are using a variable. So anytime you see a dollar sign ($) it means there is a variable.
Then comes the equal sign (=): it obviously means that $username_of_visitor equals ...
Followed by the value. The value is enclosed in quotes because it is a text value.
Finally we have the semicolon sign (;), which ends the instruction.


Nothing at all unless you use the "echo" function. The server has just created a variable in memory, nothing else.

Now a variable does not only contains text. it can also be assigned numbers or booleans. Take a look at how to use them.


Let me explain
Remember that a variable can contain 3 different types of "data": text, numbers and booleans. This is how they are used.

• Text : I just showed you how to do it. To write a text in a variable it must be between quotes as follows:
  Code: PHP

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  <?php $username_of_visitor = "Mateo21"; ?>

  
  
• Numbers : the only difference with text is the absence of quotes.
  
  Code: PHP

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  <?php $number_of_brothers = 3; ?>

  
  
By doing so, PHP understands that it is a number and not a text. You therefore have to remember that if you want to store a number you do not need to put quotes
• Booleans : I guess most of you knew about text and numbers (At least I hope ).
  
  But booleans are probably new for you. A boolean expresses a truth value. It can be either true or false
  
  In PHP use the keyword true or false to specify a variable. Do not use quotes. Example :
  
  Code: PHP

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  <?php $I_am_a_zero = true; $I_am_good_in_php = false; ?>

  
  
  In this example, I have created two booleans so that you can see the two possiblities $I_am_a_zero = true $I_am_good_in_php = false
  You want to know what booleans are used for? I won't tell you now. I will let you discover in the chapters on conditions.
  

To sum up: there are 3 types of elements in a variable, text numbers and booleans. Text is between quotes and for numbers and booleans, there are no quotes. Now that you've got this let's move on

Displaying the value of a variable

Now we are going to do a small experiment: use the echo function with variables. It is very easy as you will see.
Code: PHP

What does it display? Wonderful, magic!! it displays Mateo21! Now try it by yourself to see if it works. Change the value of the variable in the first line and you will have something else displayed. It seems to be so simplistic, but you have to do it in order to understand the functioning of variables. The echo instruction is used to display the content of a variable, but you are not obliged to do so. Here is a very useful example: Code: PHP

You notice that the echo instruction has displayed our text with the $username_of_visitor inserted in. When the PHP page is generated, $username_of_visitor will display what it contains which in our case is Welcome Mateo21! You can now try by yourself and insert in your text the value of 1, 2, 3 variables... You will often need this easy technic in the next chapters, so practice it!

Now, let's have your computer do some exercises. You will notice that arithmetic operations are just like a game. If you didn't know it let me tell PHP allows you to do arithmetic operations. Don't be scared because we are not going to do complex operations but just additions, substractions, multiplications and divisions. Not too difficult I hope? Here we are going to work on variables containing numbers only. Here are the symbols to know for calculations.

Operator	description
+	Add two numbers together
-	Substract the second number from the first number
*	Multiply two numbers together
/	Divide the first number by the second

The rest is just a game.

Code: PHP

Come on guys! This is not complicated. Check my calculations! As you can see, it's all easy You should not be scared to play with variables. Take a look at the following operations made with multiple variables.

Code: PHP

This is pure logic and I can't tell you more. If you understand all these codes then you've made it and you are a professional of variables

Another interesting aspect of PHP is that we can use it to pass on variables from one page to another. You will see how useful it is for instance when passing on visitors names. I told you that once the page is generated, variables are removed. But how can we retrieve their value on another page?

Modify address during transfer

You have certainly seen this a couple of times but you have never asked yourself why some addresses were so long? For instance

http://www.mysite.com/infos.php?day=27&month=07&year=2003&title=Information

Well the answer is simple. It's because this is the place where your variables are and that's the way you can retrieve their value.


Well, this is pure HTML. As you already know, to create a link we use the <a> tag. For example:

Code: HTML

At the end of infos.php, use a question mark(?). Then write the name of the variable, an equal sign and the matching value:

http://www.mysite.com/infos.php?jour=27

This manipulation will create a particular type of variable: $_GET['day'] with the value 27 !
And if you want to create more variables, you just need to separate them by &. Note that in your HTML code you should not use the & symbol on your keyboard but instead use the corresponding HTML code &amp;
Look at the following example:

http://www.mysite.com/infos.php?day=27&amp;month=07&amp;year=2003&amp;title=Information

All the &amp; have been transformed into & by the visitor's browser.
In this case, four variables are created which correspond to the following instructions:

• $_GET['day'] = 27;
• $_GET['month'] = 07;
• $_GET['year'] = 2003;
• $_GET['title'] = "Information";

These variables seems a bit confusing but don't stop here! We are going to do a small experiment to see in practical terms what it is. We need 2 pages for that.

• The page with the link (<a href="...">)
• And the page where we retrieve our variables

Code: HTML

Code: PHP





So what do you think? Interesting isn't it?
You are now discovering a great aspect of PHP: the source code of .php target is very small, yet the page displays a different output everytime. The .php target page can display anything you want and you don't need to change its code!

Don't hesitate to do your practise by yourself so that you get used to this way of passing on variables.

Using a form to pass on variables.

There is also another practical way to pass on variables. You can do so by using a form (a form with text zones, cells, hits links etc.)

There is an entire chapter on forms in part III (I know we are only in the first part ). Forms cover a very wide domain and deal with some complicated things.
I don't want to confuse you, so we are going to see simple but useful things.

The simplest aspect is the text zone:
As you know, you can write anything in it. Our objective is to retrieve what the visitor has written.

We are going to follow the procedure we used before: a source .php page (with the text zone) and a target .php page (this page will display what you typed in)

Code: HTML

Code: PHP





You can now start playing with different names (Well I know you can find more funny games ). Ok let's be serious because what we are interested in is how it works?
The source .php page is a form. If you've read the chapter on (X)HTML you should know how to use it.

The only thing we can add is that "action" means "to display the .php target page". The name in the text zone is the variable's name. Here the tag is:

<input type="text" name="first name" />

Here the name of the text zone is "surname". In the target page, a $_POST['surname'] variable is created and the value is what you typed in. This is a different type of variable and I will not spend too much time explaining it here. You will understand later how it works, but right now you can start doing interesting things