PHP Global Variables - Superglobals

Superglobals were introduced in PHP 4.1.0, and are built-in variables that are always available in all scopes.

PHP Global Variables - Superglobals

Some predefined variables in PHP are "superglobals", which means that they are always accessible, regardless of scope - and you can access them from any function, class or file without having to do anything special.

The PHP superglobal variables are:

  • $GLOBALS
  • $_SERVER
  • $_REQUEST
  • $_POST
  • $_GET
  • $_FILES
  • $_ENV
  • $_COOKIE
  • $_SESSION

Let's learn more about these superglobal variables.

$GLOBALS

$GLOBALS is an array that contains all global variables.

Global Variables

Global variables are variables that can be accessed from any scope.

Variables of the outer most scope are automatically global variables, and can be used by any scope, e.g. inside a function.

To use a global variable inside a function you have to either define them as global with the global keyword, or refer to them by using the $GLOBALS syntax.

Example

Refer to the global variable $x inside a function:

$x = 75;
  
function myfunction() {
  echo $GLOBALS['x'];
}

myfunction()

This is different from other programming languages where global variables are available without specifically referring to them as global.

Example

In PHP you get nothing (or an error) when referring to a global variable without the $GLOBALS syntax:

$x = 75;
  
function myfunction() {
  echo $x;
}

myfunction()

You can also refer to global variables inside functions by defining them as global with the global keyword.

Example

Define $x as global inside a function:

$x = 75;
  
function myfunction() {
  global $x;
  echo $x;
}

myfunction()

Create Global Variables

Variables created in the outer most scope are global variables either if they are created using the $GLOBALS syntax or not:

Example

$x = 100;

echo $GLOBALS["x"];
echo $x;

Variables created inside a function belongs only to that function, but you can create global variables inside a function by using the $GLOBALS syntax:

Example

Create a global variable from inside a function, and use it outside of the function:

$_SERVER

$_SERVER is a PHP super global variable which holds information about headers, paths, and script locations.

The example below shows how to use some of the elements in $_SERVER:

Example

echo $_SERVER['PHP_SELF'];
echo $_SERVER['SERVER_NAME'];
echo $_SERVER['HTTP_HOST'];
echo $_SERVER['HTTP_REFERER'];
echo $_SERVER['HTTP_USER_AGENT'];
echo $_SERVER['SCRIPT_NAME'];

OUTPUT

/demo/demo_global_server.php
263.185.90.66
263.185.90.66
https://scoolio.academy/
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
/demo/demo_global_server.php

The following table lists the most important elements that can go inside $_SERVER:

Element/Code Description
$_SERVER['PHP_SELF'] Returns the filename of the currently executing script
$_SERVER['GATEWAY_INTERFACE'] Returns the version of the Common Gateway Interface (CGI) the server is using
$_SERVER['SERVER_ADDR'] Returns the IP address of the host server
$_SERVER['SERVER_NAME'] Returns the name of the host server (such as www.w3schools.com)
$_SERVER['SERVER_SOFTWARE'] Returns the server identification string (such as Apache/2.2.24)
$_SERVER['SERVER_PROTOCOL'] Returns the name and revision of the information protocol (such as HTTP/1.1)
$_SERVER['REQUEST_METHOD'] Returns the request method used to access the page (such as POST)
$_SERVER['REQUEST_TIME'] Returns the timestamp of the start of the request (such as 1377687496)
$_SERVER['QUERY_STRING'] Returns the query string if the page is accessed via a query string
$_SERVER['HTTP_ACCEPT'] Returns the Accept header from the current request
$_SERVER['HTTP_ACCEPT_CHARSET'] Returns the Accept_Charset header from the current request (such as utf-8,ISO-8859-1)
$_SERVER['HTTP_HOST'] Returns the Host header from the current request
$_SERVER['HTTP_REFERER'] Returns the complete URL of the current page (not reliable because not all user-agents support it)
$_SERVER['HTTPS'] Is the script queried through a secure HTTP protocol
$_SERVER['REMOTE_ADDR'] Returns the IP address from where the user is viewing the current page
$_SERVER['REMOTE_HOST'] Returns the Host name from where the user is viewing the current page
$_SERVER['REMOTE_PORT'] Returns the port being used on the user's machine to communicate with the web server
$_SERVER['SCRIPT_FILENAME'] Returns the absolute pathname of the currently executing script
$_SERVER['SERVER_ADMIN'] Returns the value given to the SERVER_ADMIN directive in the web server configuration file (if your script runs on a virtual host, it will be the value defined for that virtual host) (such as [email protected])
$_SERVER['SERVER_PORT'] Returns the port on the server machine being used by the web server for communication (such as 80)
$_SERVER['SERVER_SIGNATURE'] Returns the server version and virtual host name which are added to server-generated pages
$_SERVER['PATH_TRANSLATED'] Returns the file system based path to the current script
$_SERVER['SCRIPT_NAME'] Returns the path of the current script
$_SERVER['SCRIPT_URI'] Returns the URI of the current page

$_REQUEST

$_REQUEST is a PHP super global variable which contains submitted form data, and all cookie data.

In other words, $_REQUEST is an array containing data from $_GET, $_POST, and $_COOKIE.

You can access this data with the $_REQUEST keyword followed by the name of the form field, or cookie, like this:

$_REQUEST['firstname']

Using $_REQUEST on $_POST Requests

POST request are usually data submitted from an HTML form.

Here is an example of how a HTML form could look like:

HTML form

<html>
<body>

<form method="post" action="demo_request.php">
  Name: <input type="text" name="fname">
  <input type="submit">
</form>

</body>
</html>

When a user clicks the submit button, the form data is sent to a PHP file specified in the action attribute of the <form> tag.

In the action file we can use the $_REQUEST variable to collect the value of the input field.

PHP file

$name = $_REQUEST['fname'];
echo $name;

In the example below we have put the HTML form and PHP code in the same PHP file.

We have also added some extra lines for security.

Example

<html>
<body>

<form method="post" action="<?php echo $_SERVER['PHP_SELF'];?>">
  Name: <input type="text" name="fname">
  <input type="submit">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $name = htmlspecialchars($_REQUEST['fname']);
  if (empty($name)) {
    echo "Name is empty";
  } else {
    echo $name;
  }
}
?>

</body>
</html>

Using $_REQUEST on $_GET Requests

GET request can be form submissions as in the example above, with the method attribute of the HTML <form> element set to GET.

GET requests can also be data from a query string (information added after a URL address).

Here is an example of how an HTML hyperlink, with a query string could look like:

HTML link

<html>
<body>

<a href="demo_phpfile.php?subject=PHP&web=scoolio.academy">Test $GET</a>

</body>
</html>

When a user clicks the link, the query string data is sent to demo_phpfile.php.

In the PHP file we can use the $_REQUEST variable to collect the value of the query string.

Example

The PHP file demo_phpfile.php:

<html>
<body>

<?php
echo "Study " . $_REQUEST['subject'] . " at " . $_REQUEST['web'];
?>

</body>
</html>

$_POST

$_POST contains an array of variables received via the HTTP POST method.

There are two main ways to send variables via the HTTP Post method:

  • HTML forms
  • JavaScript HTTP requests

$_POST in HTML Forms

A HTML form submits information via the HTTP POST method if the form's method attribute is set to "POST".

To demonstrate this, we start by creating a simple HTML form:

HTML Form

<html>
<body>

<form method="POST" action="demo_request.php">
  Name: <input type="text" name="fname">
  <input type="submit">
</form>

</body>
</html>

When a user clicks the submit button, the form data is sent to a PHP file specified in the action attribute of the <form> tag.

In the action file we can use the $_POST variable to collect the value of the input field.

PHP file

$name = $_POST['fname'];
echo $name;

In the example below we have put the HTML form and PHP code in the same PHP file.

We have also added some extra lines for security.

Example

<html>
<body>

<form method="POST" action="<?php echo $_SERVER['PHP_SELF'];?>">
  Name: <input type="text" name="fname">
  <input type="submit">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $name = htmlspecialchars($_POST['fname']);
  if (empty($name)) {
    echo "Name is empty";
  } else {
    echo $name;
  }
}
?>

</body>
</html>

$_POST in JavaScript HTTP Requests

When sending a HTTP request in JavaScript, you can specify that the HTTP method is POST.

To demonstrate this we start by creating a JavaScript function containing a HTTP request:

JavaScript function

function myfunction() {
  const xhttp = new XMLHttpRequest();
  xhttp.open("POST", "demo_phpfile.php");
  xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhttp.onload = function() {
    document.getElementById("demo").innerHTML = this.responseText;
  }
  xhttp.send("fname=Mary");
  }
}

The code above will:

  1. Intiate a HTTP request
  2. Set the HTTP method to POST
  3. Set a valid request header
  4. Create a function to execute when the request is done
  5. Send the HTTP request with a variable fname set to Mary

Look at the function that will be executed when the request is done:

xhttp.onload = function() {
    document.getElementById("demo").innerHTML = this.responseText;
  }

It will try to write a response from the operation in a HTML element with id="demo".

Let us make a HTML page with such element, and also a button that executes the function.

If we also add the the JavaScript, the page looks like this:

Example

How to post and receive data from a HTTP request:

<html>
<script>
function myfunction() {
  const xhttp = new XMLHttpRequest();
  xhttp.open("POST", "demo_ajax.php");
  xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhttp.onload = function() {
    document.getElementById("demo").innerHTML = this.responseText;
  }
  xhttp.send("fname=Mary");
  }
}
</script>
<body>

<button onclick="myfunction()">Click me!</button>

<h1 id="demo"></h1>

</body>
</html>

In the PHP file that receive this HTTP request (demo_ajax.php), we simply use the $_POST variable to retrieve the fname variable, and writes it as a response.

PHP file

$name = $_POST['fname'];
echo $name;

$_GET

$_GET contains an array of variables received via the HTTP GET method.

There are two main ways to send variables via the HTTP GET method:

  • Query strings in the URL
  • HTML Forms

Query string in the URL

A query string is data added at the end of a URL. In the link below, everything after the ? sign is part of the query string:

<a href="demo_phpfile.php?subject=PHP&web=scoolio.academy">Test $GET</a>

The query string above contains two key/value pairs:

subject=PHP
web=scoolio.academy

In the PHP file we can use the $_GET variable to collect the value of the query string.

Example

The PHP file demo_phpfile.php:

<html>
<body>

<?php
echo "Study " . $_GET['subject'] . " at " . $_GET['web'];
?>

</body>
</html>

$_GET in HTML Forms

A HTML form submits information via the HTTP GET method if the form's method attribute is set to "GET".

To demonstrate this, we start by creating a simple HTML form:

HTML Form

<html>
<body>

<form action="welcome_get.php" method="GET">
  Name: <input type="text" name="name">
  E-mail: <input type="text" name="email">
  <input type="submit">
</form>

</body>
</html>

When a user clicks the submit button, the form data is sent to a PHP file specified in the action attribute of the <form> tag.

The form fields are sent to the PHP file, with your input, as query strings:

welcome_get.php?name=John&[email protected]

In the action file we can use the $_GET variable to collect the value of the input fields.

Example

PHP code inside the welcome_get.php page:

<html>
<body>

Welcome <?php echo $_GET["name"]; ?><br>
Your email address is: <?php echo $_GET["email"]; ?>

</body>
</html>