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PHP 5 Variables
 
Variables are "containers" for storing information.
 
Creating (Declaring) PHP Variables
 
In PHP, a variable starts with the $ sign, followed by the name of the variable:
 
Example
  1. <?php
  2. $txt = "Hello world!";
  3. $x = 5;
  4. $y = 10.5;
  5. ?>
 
After the execution of the statements above, the variable $txt will hold the value Hello world!, the variable $x will hold the value 5, and the variable $y will hold the value 10.5.
 
Note: When you assign a text value to a variable, put quotes around the value.
 
Note: Unlike other programming languages, PHP has no command for declaring a variable. It is created the moment you first assign a value to it.
 
Think of variables as containers for storing data.

 
PHP Variables
 
A variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume).
 
Rules for PHP variables:
 
  • A variable starts with the $ sign, followed by the name of the variable
  • A variable name must start with a letter or the underscore character
  • A variable name cannot start with a number
  • A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
  • Variable names are case-sensitive ($age and $AGE are two different variables)
     
Remember that PHP variable names are case-sensitive!

 
Output Variables
 
The PHP echo statement is often used to output data to the screen.
 
The following example will show how to output text and a variable:

 
Example
  1. <?php
  2. $txt = "W3Schools.com";
  3. echo "I love $txt!";
  4. ?>
 
The following example will produce the same output as the example above:

 
Example
  1. <?php
  2. $txt = "W3Schools.com";
  3. echo "I love " . $txt . "!";
  4. ?>
 
The following example will output the sum of two variables:

 
Example
  1. <?php
  2. $x = 5;
  3. $y = 4;
  4. echo $x + $y;
  5. ?>
 
Note: You will learn more about the echo statement and how to output data to the screen in the next chapter.

 
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 other languages such as C, C++, and Java, the programmer must declare the name and type of the variable before using it.
 
 
PHP Variables Scope
 
In PHP, variables can be declared anywhere in the script.
 
The scope of a variable is the part of the script where the variable can be referenced/used.
 
PHP has three different variable scopes:
  • local
  • global
  • static
 
Global and Local Scope
 
A variable declared outside a function has a GLOBAL SCOPE and can only be accessed outside a function:

 
Example
  1. <?php
  2. $x = 5; // global scope
  3. function myTest() {
  4.     // using x inside this function will generate an error
  5.     echo "<p>Variable x inside function is: $x</p>";
  6. myTest();
  7. echo "<p>Variable x outside function is: $x</p>";
  8. ?>
 
 
A variable declared within a function has a LOCAL SCOPE and can only be accessed within that function
:
 
Example
  1. <?php
  2. function myTest() {
  3.     $x = 5; // local scope
  4.     echo "<p>Variable x inside function is: $x</p>";
  5. myTest();
  6. // using x outside the function will generate an error
  7. echo "<p>Variable x outside function is: $x</p>";
  8. ?>
 
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.

 
PHP The global Keyword
 
The global keyword is used to access a global variable from within a function.
 
To do this, use the global keyword before the variables (inside the function):

 
Example
  1. <?php
  2. $x = 5;
  3. $y = 10;
  4. function myTest() {
  5.     global $x, $y;
  6.     $y = $x + $y;
  7. }
  8. myTest();
  9. echo $y; // outputs 15
  10. ?>
 
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
  1. <?php
  2. $x = 5;
  3. $y = 10;
  4. function myTest() {
  5.     $GLOBALS['y'] = $GLOBALS['x'] + $GLOBALS['y'];
  6. myTest();
  7. echo $y; // outputs 15
  8. ?>
 
 
PHP The static Keyword
 
Normally, when a function is completed/executed, all of its variables are deleted. However, sometimes we want a local variable NOT to be deleted. We need it for a further job.
 
To do this, use the static keyword when you first declare the variable:

 
Example
  1. <?php
  2. function myTest() {
  3.     static $x = 0;
  4.     echo $x;
  5.     $x++;
  6. }
  7. myTest();
  8. myTest();
  9. myTest();
  10. ?>
 
Then, each time the function is called, that variable will still have the information it contained from the last time the function was called.
 
Note: The variable is still local to the function.


2016-12-08
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