When to Use PHP’s ?: vs ?? Operators: A Complete Guide

Understanding the two operators

In PHP you often need to provide a default value when a variable may be empty. The ternary shorthand ?: and the null‑coalescing operator ?? both help, but they behave differently.

1. Ternary shorthand ?:

Equivalent to $result = $a ? $a : $default;. It evaluates $a in a boolean context; if the value is truthy it is returned, otherwise $default is used. $result = $a ?: $default; Key points

Truthiness includes true, any non‑zero number, non‑empty string, non‑empty array, etc.

Falsy values ( 0, 0.0, "", '0', false, null, undefined variable, empty array) cause the default to be used.

Using an undefined variable triggers an E_NOTICE warning.

Example

$username = $_POST['username'] ?: 'Anonymous User'; // uses posted value if set and non‑empty
$count = 0;
echo $count ?: 'No data'; // outputs "No data" because 0 is falsy

2. Null‑coalescing operator ??

Introduced in PHP 7. Syntax: $result = $a ?? $default;. It checks whether $a is set and not null. If so, it returns $a; otherwise it returns $default. Other falsy values are considered valid. $result = $a ?? $default; Key points

No notice is raised for undefined variables.

Only null or an undefined variable trigger the default.

Values such as 0, false, "" are returned unchanged.

Example

// $_GET['page'] is not set
$page = $_GET['page'] ?? 1; // $page becomes 1

$count = 0;
echo $count ?? 'No data'; // outputs 0

$name = null;
$user = $name ?? 'Unknown User'; // $user becomes "Unknown User"

Core comparison

Evaluation criteria : ?: uses truthiness; ?? uses “isset and not null”.

Handling of null : Both fall back to the default, but ?? does so only for null or undefined.

Handling of 0 , empty string, false : ?: treats them as falsy and uses the default; ?? returns them.

Undefined variables : ?: raises E_NOTICE; ?? is safe.

PHP version : ?: works from PHP 5.3+; ?? requires PHP 7.0+.

Practical scenarios

Use ?? when reading from superglobals or any source where the key may be missing:

$page = $_GET['page'] ?? 1;
$searchTerm = $_POST['keyword'] ?? '';

Use ?? for values that may be null from functions or database calls:

$userEmail = $user->getEmail() ?? '[email protected]';

Use ?: when you want to treat any falsy value as “empty”, for example configuration flags:

$shouldDisplay = $config['display_errors'] ?: false;

Chaining

Null‑coalescing chain returns the first defined, non‑null value: $result = $a ?? $b ?? $c ?? 'final-default'; Ternary shorthand chain returns the first truthy value:

$result = $a ?: $b ?: $c ?: 'final-default';

Conclusion

In modern PHP code, ?? is generally safer for handling user input, object properties, or any data that may be missing, because it preserves legitimate falsy values such as 0 or false. Use ?: only when you intentionally want to replace all falsy values with a default.