Variables are “containers” for storing information.
Do You Remember Algebra From School?
Do you remember algebra from school? x=5, y=6, z=x+y
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).
As with algebra, PHP variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carName.
Rules for PHP variable names:
- Variables in PHP starts with a $ sign, followed by the name of the variable
- The variable name must begin with a letter or the underscore character
- A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
- A variable name should not contain spaces
- Variable names are case sensitive (y and Y are two different variables)
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:
After the execution of the statement above, the variable myCar will hold the value Volvo.
Tip: If you want to create a variable without assigning it a value, then you assign it the value of null.
Let’s create a variable containing a string, and a variable containing a number:
Note: When you assign a text value to a variable, put quotes around the value.
PHP is a Loosely Typed Language
In PHP, a variable does not need to be declared before adding a value to it.
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, you have to declare (define) the type and name of the variable before using it.
PHP Variable Scope
The scope of a variable is the portion of the script in which the variable can be referenced.
PHP has four different variable scopes:
A variable declared within a PHP function is local and can only be accessed within that function. (the variable has local scope):
$a = 5; // global scope
echo $a; // local scope
The script above will not produce any output because the echo statement refers to the local scope variable $a, 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 refers to any variable that is defined outside of any function.
Global variables can be accessed from any part of the script that is not inside a function.
To access a global variable from within a function, use the global keyword:
$a = 5;
$b = 10;
global $a, $b;
$b = $a + $b;
The script above will output 15.
PHP also stores all global variables in an array called $GLOBALS[index]. Its index is 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 as this:
$a = 5;
$b = 10;
$GLOBALS[‘b’] = $GLOBALS[‘a’] + $GLOBALS[‘b’];
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:
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.
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:
// function code
Parameters are also called arguments. We will discuss them in more detail when we talk about functions.