How to Declare Global Variable in PHP

1. Use the global Keyword to Declare a Global Variable in a Local Scope in PHP
2. Use the $GLOBALS Super Global Variable to Use the Global Variable in the Local Scope in PHP
3. Use the define() Function to Define a Constant Global Variable in PHP

We will introduce a method to declare a global variable in PHP using the global keyword. This method will set the global scope of the variable declared outside of a function to the local scope to use the variable inside the function.

This article will demonstrate another method to declare a global variable in PHP using the $GLOBALS super global variable. This method will use the super global variable to access the global variable in a local scope.

We will also demonstrate another method to declare a global variable in PHP using the define() function. This method is a way of declaring a constant global variable in PHP. We cannot change the value of the constant.

Use the global Keyword to Declare a Global Variable in a Local Scope in PHP

We can use the global keyword to declare a global variable in a local scope in PHP. The global keyword sets the global scope as a variable to the local scope. We can define a variable outside a function. When we try to access the variable inside the function, the global scope of the variable won’t let us. So we can use the global keyword inside the function with the variable. Then, we can access the variable inside the function.

For example, create a variable $crypto and assign Bitcoin to it. Then, create a function body(). Inside the function, use the global keyword before the $crypto variable. Print the $crypto variable using the echo statement and concatenate the string is a top cryptocurrency. using the . dot operator. Outside the function body, call the body() function.

The example below imports the global scope of the $crypto variable to the local scope inside the body() function. Thus, we can access the $crypto variable inside the function. When we try to print the variable without using the global keyword, we won’t be able to do that because the local scope of the variable does not exist. Check the PHP Manual to learn more about the scopes and the global keyword.

Example Code:

#php 7.x
<?php
$crypto = 'Bitcoin';
function body(){
    global $crypto;
    echo $crypto." is a top cryptocurrency.";
}
body();
?>

Output:

Bitcoin is a top cryptocurrency.

Use the $GLOBALS Super Global Variable to Use the Global Variable in the Local Scope in PHP

We can use the $GLOBALS super global variable to reference the global scope variables. The $GLOBALS variable is an associative array that contains the reference of the variables defined in the global scope. We can write the variable inside the $GLOBALS brackets to reference the global variable as $GLOBALS["name"]. This method attempts to print a local variable and a global variable inside a function. It uses the $GLOBALS super global variable to reference a variable in a global scope inside the function.

For example, create a variable $var and assign a string global scope variable. Write a function body() and create the same variable $var inside the function and assign the string local scope variable this time. Firstly, echo the $var variable referencing it in the $GLOBALS array as $GLOBALS["var"]. In the next line, print the $var variable with the echo statement.

The example below prints the $var variables two times. The first one uses the $GLOBALS["var"] array, and as a result, it prints the global variable defined outside the function. In the second time, the variable inside the function displays. Check the PHP Manual to know more about the $GLOBALS super global variable.

Code Example:

#php 7.x
<?php
$var = "global scope variable";
function body() {
    $var = "local scope variable";
    echo '$var in global scope: ' . $GLOBALS["var"] . "<br>";
    echo '$var in current scope: ' . $var ;
}
body();
?>

Output:

$var in global scope: global scope variable 
$var in current scope: local scope variable

Use the define() Function to Define a Constant Global Variable in PHP

This method uses the define() function to define a global variable in PHP. The function takes two parameters. The first parameter is the constant name, and the second is the value of the constant. The constant is case-insensitive by default. We can access the constant from anywhere in the script. There is no use of the $ sign while defining a constant. As the name stands, its value is immutable. The constant only can hold the string and number as the value.

For example, write a define() function and give the name of the constant as BAND and the value as Opeth. Create a function bandName() and use the echo statement to display the BAND constant. Outside the function, call the function bandName(). The script will output the value Opeth. Check the PHP Manual to learn more about the define() function.

Example Code:

# php 7.x
<?php
define('BAND', 'Opeth');
function bandName()
{
    echo BAND;
}
bandName();
?>

Output:

Opeth