PHP AND VARIABLES

What is PHP?

• PHP stands for PHP: Hypertext Preprocessor
• PHP is a widely-used, open source scripting language
• PHP scripts are executed on the server
• PHP is free to download and use

What is a PHP File?

• PHP files can contain text, HTML, JavaScript code, and PHP code
• PHP code are executed on the server, and the result is returned to the browser as plain HTML
• PHP files have a default file extension of ".php"

What Can PHP Do?

• PHP can generate dynamic page content
• PHP can create, open, read, write, and close files on the server
• PHP can collect form data
• PHP can send and receive cookies
• PHP can add, delete, modify data in your database
• PHP can restrict users to access some pages on your website
• PHP can encrypt data
• With PHP you are not limited to output HTML. You can output images, PDF files, and even Flash movies. You can also output any text, such as XHTML and XML.

Why PHP?

• PHP runs on different platforms (Windows, Linux, Unix, Mac OS X, etc.)
• PHP is compatible with almost all servers used today (Apache, IIS, etc.)
• PHP has support for a wide range of databases
• PHP is easy to learn and runs efficiently on the server side
• Creating (Declaring) PHP Variables
• PHP has no command for declaring a variable.
• A variable is created the moment you first assign a value to it:
• $txt="Hello world!";
• $x=5;
• After the execution of the statements above, the variable txt will hold the value Hello world!, and the variable xwill hold the value 5.
• Note: When you assign a text value to a variable, put quotes around the value.
•  
• PHP is a Loosely Typed Language
• In the example above, notice that we did not have to tell PHP which data type the variable is.
• PHP automatically converts the variable to the correct data type, depending on its value.
• In a strongly typed programming language, we will have to declare (define) the type and name of the variable before using it.

PHP Variable Scopes

The scope of a variable is the part of the script where the variable can be referenced/used.

PHP has four different variable scopes:     

1. local
2. global
3. static
4. parameter

Local Scope

A variable declared within a PHP function is local and can only be accessed within that function:

Example

$x=5; // global scope

function myTest()
{
echo $x; // local scope
}

myTest();
?>

The script above will not produce any output because the echo statement refers to the local scope variable $x, which has not been assigned a value within this scope.

You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.

Local variables are deleted as soon as the function is completed.

Global Scope

A variable that is defined outside of any function, has a global scope.

Global variables can be accessed from any part of the script, EXCEPT from within a function.

To access a global variable from within a function, use the global keyword:

Example

$x=5; // global scope
$y=10; // global scope

function myTest()
{
global $x,$y;
$y=$x+$y;
}

myTest();
echo $y; // outputs 15
?>

PHP also stores all global variables in an array called $GLOBALS[index]. The index holds the name of the variable.

This array is also accessible from within functions and can be used to update global variables directly.

The example above can be rewritten like this:

Example

$x=5;
$y=10;

function myTest()
{
$GLOBALS['y']=$GLOBALS['x']+$GLOBALS['y'];
}

myTest();
echo $y;
?>

Static Scope

When a function is completed, all of its variables are normally deleted. However, sometimes you want a local variable to not be deleted.

To do this, use the static keyword when you first declare the variable:

Example

function myTest()
{
static $x=0;
echo $x;
$x++;
}

myTest();
myTest();
myTest();

?>

Note: Then, each time the function is called, that variable will still have the information it contained from the last time the function was called. The variable is still local to the function.

Parameter Scope

A parameter is a local variable whose value is passed to the function by the calling code.

Parameters are declared in a parameter list as part of the function declaration:

Example

function myTest($x)
{
echo $x;
}

myTest(5);

?>

Note: Parameters are also called arguments