Variables are "containers" for storing information:
Example
<?php
$x=5;
$y=6;
$z=$x+$y;
echo $z;
?>
Much Like Algebra
x=5
y=6
z=x+y
In algebra we use letters (like x) to hold values (like 5).
From the expression z=x+y above, we can calculate the value of z to be 11.
In PHP these letters are called
variables.
|
Think of variables as containers for storing data. |
PHP Variables
As with algebra, PHP variables can be used to hold values (x=5) or expressions (z=x+y).
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 ($y and $Y are two different variables)
|
Remember that PHP variable names are case-sensitive! |
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:
Example
<?php
$txt="Hello world!";
$x=5;
$y=10.5;
?>
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.
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 and Global Scope
A variable declared
outside a function has a GLOBAL SCOPE and can only be accessed
outside a function.
A variable declared
within a function has a LOCAL SCOPE and can only
be accessed within that function.
The following example tests variables with local and global scope:
Example
<?php
$x=5; // global scope
function myTest() {
$y=10; //
local scope
echo "<p>Test variables inside the function:</p>";
echo
"Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest();
echo "<p>Test variables outside the function:</p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
In the example above there are two variables $x and $y and a function
myTest(). $x is a global variable since it is declared outside the function and
$y is a local variable since it is created inside the function.
When we output the values of the two variables inside the myTest() function,
it prints the value of $y as it is the locally declared, but cannot print the
value of $x since it is created outside the function.
Then, when we output the values of the two variables outside the myTest()
function, it prints the value of $x, but cannot print the value of $y since it
is a local variable and it is created inside the myTest() function.
|
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
<?php
$x=5;
$y=10;
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
<?php
$x=5;
$y=10;
function myTest()
{
$GLOBALS['y']=$GLOBALS['x']+$GLOBALS['y'];
}
myTest();
echo $y; // outputs 15
?>
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
<?php
function myTest()
{
static $x=0;
echo $x;
$x++;
}
myTest();
myTest();
myTest();
?>
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.
