Variables are "containers" for storing information:
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.|
As with algebra, PHP variables can be used to hold values (x=5) or expressions (z=x+y).
Variable can have short names (like x and y) or more descriptive names (age, carname, totalvolume).
Rules for PHP variables:
|Both PHP statements and PHP variables are case-sensitive.|
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 statements above, the variable txt will hold the value Hello world!, and the variable x will hold the value 5.
Note: When you assign a text value to a variable, put quotes around the value.
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.
The scope of a variable is the part of the script where the variable can be referenced/used.
PHP has four different variable scopes:
A variable declared within a PHP function is local and can only be accessed within that function:
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.
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:
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:
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:
Parameters are also called arguments. We will discuss it in more details in our PHP functions chapter.
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