PHP - Null Coalescing Operator

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The Null Coalescing operator is one of the many new features introduced in PHP 7. The word "coalescing" means uniting many things into one. This operator is used to replace the ternary operation in conjunction with the isset() function.

Ternary Operator in PHP

PHP has a ternary operator represented by the "?" symbol. The ternary operator compares a Boolean expression and executes the first operand if it is true, otherwise it executes the second operand.

expr ? statement1 : statement2;

Example

Let us use the ternary operator to check if a certain variable is set or not with the help of the isset() function, which returns true if declared and false if not.

<?php
   $x = 1;
   $var = isset($x) ? $x : "not set";
   echo "The value of x is $var";
?>

It will produce the following output −

The value of x is 1

Now, let's remove the declaration of "x" and rerun the code −

<?php
   # $x = 1;
   $var = isset($x) ? $x : "not set";
   echo "The value of x is $var";
?>

The code will now produce the following output −

The value of x is not set

The Null Coalescing Operator

The Null Coalescing Operator is represented by the "??" symbol. It acts as a convenient shortcut to use a ternary in conjunction with isset(). It returns its first operand if it exists and is not null; otherwise it returns its second operand.

$Var = $operand1 ?? $operand2;

The first operand checks whether a certain variable is null or not (or is set or not). If it is not null, the first operand is returned, else the second operand is returned.

Example

Take a look at the following example −

<?php
   # $num = 10;
   $val = $num ?? 0;
   echo "The number is $val";
?>

It will produce the following output −

The number is 0

Now uncomment the first statement that sets $num to 10 and rerun the code −

<?php
   $num = 10;
   $val = $num ?? 0;
   echo "The number is $val";
?>

It will now produce the following output −

The number is 10

A useful application of Null Coalescing operator is while checking whether a username has been provided by the client browser.

Example

The following code reads the name variable from the URL. If indeed there is a value for the name parameter in the URL, a Welcome message for him is displayed. However, if not, the user is called Guest.

<?php
   $username = $_GET['name'] ?? 'Guest';
   echo "Welcome $username";
?>

Assuming that this script "hello.php" is in the htdocs folder of the PHP server, enter http://localhost/hello.php?name=Amar in the URL, the browser will show the following message −

Welcome Amar

If http://localhost/hello.php is the URL, the browser will show the following message −

Welcome Guest

The Null coalescing operator is used as a replacement for the ternary operator’s specific case of checking isset() function. Hence, the following statements give similar results −

<?php
   $username = isset($_GET['name']) ? $_GET['name'] : 'Guest';
   echo "Welcome $username";
?>

It will now produce the following output −

Welcome Guest

You can chain the "??" operators as shown below −

<?php
   $username = $_GET['name'] ?? $_POST['name'] ?? 'Guest';
   echo "Welcome $username";
?>

It will now produce the following output −

Welcome Guest

This will set the username to Guest if the variable $name is not set either by GET or by POST method.