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PHP Elvis vs Null Coalescing: Which to Use?

By Filip on 10/31/2024

Learn the differences between PHP's shorthand ternary (Elvis) operator and the null coalescing operator for concise and efficient conditional assignments.

PHP Elvis vs Null Coalescing: Which to Use?

Table of Contents

Introduction

The Elvis operator (?:) and the null coalescing operator (??) provide concise ways to manage potentially undefined variables in PHP, making your code cleaner and less error-prone.

Step-by-Step Guide

The Elvis operator (?:) and the null coalescing operator (??) are both shorthand ways to handle potentially undefined variables in PHP.

Let's say you have a variable $name that might not have a value:

$name = null;

Elvis Operator (?:)

The Elvis operator checks if the expression before it is truthy. If it is, it returns the value of that expression. If not, it returns the value after the colon.

$greeting = $name ?: "Stranger"; 
// $greeting will be "Stranger" because $name is null (which is falsy)

Null Coalescing Operator (??)

The null coalescing operator specifically checks if the variable before it is not null. If it's not null, it returns the variable's value. Otherwise, it returns the value after the operator.

$greeting = $name ?? "Stranger"; 
// $greeting will be "Stranger" because $name is null

Key Differences

  • Type of Check: Elvis checks for "truthiness" (which includes null, 0, '', etc.), while null coalescing specifically checks for null.
  • Strictness: Null coalescing is stricter in that it only cares about null.

When to Use Each

  • Elvis (?:): Use when you want a default value if the expression is falsy for any reason (empty string, zero, etc.).
  • Null Coalescing (??): Use when you specifically want a default value if a variable is null.

Example

$data = ['age' => 25];

// Elvis: Checks if $data['name'] exists and is truthy
$name = $data['name'] ?: 'Unknown'; 

// Null Coalescing: Checks if $data['age'] is specifically null
$age = $data['age'] ?? 18;

Code Example

The PHP code demonstrates the Elvis operator (?:) and the null coalescing operator (??) for providing default values. It shows how the Elvis operator checks for truthiness, while the null coalescing operator specifically checks for null. Examples illustrate their behavior with different variable types and conditions, including null, falsy values, and array elements. The code highlights the differences in their checks and provides practical use cases for each operator.

<?php

// Example with Elvis operator (?:)
$name = null;
$greeting = $name ?: "Stranger"; 
echo "Elvis: Hello, " . $greeting . "!\n"; // Output: Hello, Stranger!

$age = 0; // 0 is considered falsy
$displayAge = $age ?: 25; // Using Elvis operator to provide a default value
echo "Elvis: Your age is: " . $displayAge . "\n"; // Output: Your age is: 25

// Example with Null Coalescing operator (??)
$city = null;
$location = $city ?? "Unknown City";
echo "Null Coalescing: You are from " . $location . "\n"; // Output: You are from Unknown City

$country = "USA"; // Non-null value
$residence = $country ?? "Unknown Country";
echo "Null Coalescing: You live in " . $residence . "\n"; // Output: You live in USA

// Demonstrating the difference in checks
$value = ""; // Empty string is considered falsy
$resultElvis = $value ?: "Default Value";
$resultNullCoalescing = $value ?? "Default Value";

echo "Elvis: " . $resultElvis . "\n"; // Output: Default Value (because empty string is falsy)
echo "Null Coalescing: " . $resultNullCoalescing . "\n"; // Output:  (empty string is not null)

// Practical example with an array
$data = ['name' => 'Alice', 'age' => 30];

// Using Elvis operator - provides a default if the value is falsy
$username = $data['username'] ?: 'Guest'; 
echo "Hello, " . $username . "!\n"; // Output: Hello, Guest!

// Using Null Coalescing operator - provides a default only if the value is null
$userAge = $data['age'] ?? 18; 
echo "Your age is: " . $userAge . "\n"; // Output: Your age is: 30 

?>

Explanation:

  1. Elvis Operator (?:) Examples:

    • We declare a variable $name as null. The Elvis operator in $greeting = $name ?: "Stranger"; checks if $name is truthy. Since null is falsy, it assigns "Stranger" to $greeting.
    • The $age variable is initialized with 0, which is considered falsy in PHP. The Elvis operator in $displayAge = $age ?: 25; assigns 25 to $displayAge because $age is falsy.

  2. Null Coalescing Operator (??) Examples:

    • The $city variable is set to null. The null coalescing operator in $location = $city ?? "Unknown City"; checks if $city is null. Since it is, it assigns "Unknown City" to $location.
    • The $country variable has a non-null value ("USA"). The null coalescing operator in $residence = $country ?? "Unknown Country"; sees that $country is not null and assigns "USA" to $residence.

  3. Difference in Checks:

    • We set $value to an empty string (""). The Elvis operator considers this falsy and assigns "Default Value" to $resultElvis. However, the null coalescing operator only checks for null, so $resultNullCoalescing remains an empty string.

  4. Practical Example with Array:

    • We have an array $data. We use the Elvis operator to get the 'username' element. Since it doesn't exist (which is considered falsy), it defaults to "Guest".
    • We use the null coalescing operator to get the 'age' element. Since it exists and is not null, it uses the value 30.

This code example clearly demonstrates the differences between the Elvis operator and the null coalescing operator, helping you understand when to use each one effectively in your PHP code.

Additional Notes

  • Performance: In terms of performance, both operators are generally very similar. The difference in execution speed is often negligible, so the choice between them should be based on the specific logic and readability requirements.
  • Readability: While both operators offer conciseness, using them excessively can sometimes hinder code readability, especially for developers unfamiliar with these shorthand notations. It's essential to strike a balance between brevity and clarity.
  • Alternatives: While powerful, these operators are not always the only solution. Traditional if statements or the isset() function can be more suitable in some scenarios, especially when more complex logic or error handling is involved.
  • PHP Version Compatibility: The null coalescing operator (??) was introduced in PHP 7.0, so it's crucial to ensure compatibility if your code needs to run on older PHP versions. The Elvis operator (?:), however, has been available since PHP 5.3.

Summary

Feature Elvis Operator (?:) Null Coalescing Operator (??)
Syntax expression ?: default_value variable ?? default_value
Check Type Evaluates the "truthiness" of an expression. Checks if a variable is specifically null.
Behavior Returns the expression's value if truthy, otherwise returns the default value. Returns the variable's value if not null, otherwise returns the default value.
Strictness Less strict, considers various falsy values (e.g., null, 0, ''). More strict, only considers null.
Use Case Provide a default value when an expression might be falsy for any reason. Provide a default value specifically when a variable might be null.
Example $name = $data['name'] ?: 'Unknown'; (Checks if $data['name'] exists and is truthy) $age = $data['age'] ?? 18; (Checks if $data['age'] is specifically null)

Conclusion

Both the Elvis operator (?:) and the null coalescing operator (??) provide concise ways to manage potentially undefined variables in PHP, making your code cleaner and less error-prone. Choose the Elvis operator when you need a default value for any falsy expression and the null coalescing operator when you specifically want a default for a null variable. Understanding their subtle differences will help you write more robust and readable PHP code.

References

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