PHP Variables Scope

Local Variables and Local Scope

In PHP, the local scope refers to variables declared inside a function or a code block, and we can only access them within that specific context.

Declaring Local Variables

To declare a local variable, we simply write the variable's name inside a function or a code block.

Example

<?php
function functionName() {
	$name = "Eloisa";
	echo "Hello, ". $name;
}

functionName();
?>

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Output

Hello, Eloisa

Explanation

In our code above, we have a function functionName() where we declare a local variable $name and assign it the value "Eloisa". Then we concatenated the variable $name with the "Hello, " string and output the result. When we call functionName(), it prints "Hello, Eloisa".

Accessing and Modifying Local Variables

We can only access local variables from within the same function or code block where they are declared. Attempting to access them from outside their scope will result in an error. Local variables can be modified by reassigning new values to them within the same scope.

Example

<?php
function increment() {
	$counter = 0;
	$counter++;
	echo $counter;
}

increment();
?>

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Output

1

Explanation

In our code above, we have a function increment() where we declare a local variable $counter and initialise it to 0. We then increment the value of the $counter by 1 and output it. When we call the increment() function, it prints 1.

Static Variables and Associated Scope

Static variables are variables that retain their value even after the function or code block where they are declared has finished executing. They are helpful when we need to persist data across multiple function calls.

Note - The term "static scope" is frequently used informally to refer to the scope in which static variables are defined. Still, it has no official definition or particular behaviour in PHP other than the scoping rules of static variables.

Declaring Static Variables

We use the static keyword before the variable name to declare a static variable.

Example

<?php
function increment() {
	static $counter = 0;
	$counter++;
	echo $counter;
}

increment();
increment();
?>

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Output

12

Explanation

In our code above, we have a function increment() where we declare a static variable $counter using the static keyword. We initialise it to 0. Inside the function, we increment the value of the $counter by 1 and output it. When we call the increment() function multiple times, it keeps track of the value of the $counter and increments it accordingly.

Accessing and Modifying Static Variables

Static variables can be accessed and used within the same function or code block where they are declared, similar to local variables. Static variables can be modified within the same function or code block where they are declared by reassigning new values to them. The updated value will be retained across multiple calls to the function.

Example

<?php
function increment() {
	static $counter = 0;
	$counter++;
	echo $counter;
}

increment();
// echo $counter;
?>

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Output

1

If we uncomment the "// echo $counter;" we will get an error.

Error: Undefined variable

Explanation

In our code above, we have a function increment() with a static variable $counter initialised to 0. Inside the function, we increment the value of the $counter by 1 and output it. When we call the increment() function, it prints 1. However, when we try to access the $counter variable outside the function, we get an error because it is not defined in the global scope.

Super Global Variables

Super global variables are predefined variables in PHP that are available in all scopes throughout the script. They are accessible from any function, class, or file without using the global keyword.

Some common examples of super global variables in PHP are:

• $_GET: Contains the query string parameters from a GET request.
   
• $_POST: Contains the form data from a POST request.
   
• $_SESSION: Contains session variables.
   
• $_COOKIE: Contains cookie variables.
   
• $_SERVER: Contains server and execution environment information.
   

Accessing Super Global Variables

Super global variables can be accessed directly without any additional declaration or keyword.

Example using $_GET:

<?php
// URL: http://something.domain/?name=Eloisa
echo $_GET['name'];
?>

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Output

Eloisa

Explanation

We accessed the $_GET super global variable in the code containing the query string parameters from a GET request. We accessed the value of the "name" parameter by using the key 'name' within the $_GET array. In this example, if the URL is "http://something.domain/?name=Eloisa", it will output "Eloisa".

Modifying Super Global Variables

We can modify the super global variables by assigning new values to their respective keys. However, modifying certain super global variables, such as $_SERVER or $_POST, is generally not recommended as they contain important system or user data.

Example using $_SESSION:

<?php
session_start();
$_SESSION['name'] = 'Eloisa';
?>

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Explanation

We started/resumed a session using session_start(). After that, we assigned $_SESSION super global variable the value 'Eloisa' to the 'name' key. This allows us to store and retrieve session-specific data. However, validating user input is essential before using super global variables to avoid security vulnerabilities.

Frequently Asked Questions

What is a global variable in PHP, and how do you declare it?

A global variable in PHP is a variable that we can access and modify from anywhere in the script. We use the global keyword to declare a global variable and write the variable's name inside or outside any function or class.

Can local variables in PHP be accessed outside of their function?

No, local variables in PHP have a limited scope. We can only access them within the function where they are defined.

What is the scope of variables in PHP?

In PHP, the different program parts where the variable is accessible is the scope of that variable.

What are static variables in PHP?

Static variables in PHP are local variables that retain their values between function calls. They are declared using the static keyword.

Conclusion

In this article, we explored the PHP variables scope. We covered global and local variables, static variables, and super-global variables. By understanding the scope of variables, you can effectively manage data in your PHP applications and create efficient code. Alright! So now that you have learnt about PHP variables scope, you can also refer to other similar articles.

• Introduction to PHP
• PHP variables
• Scope of Variable

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