PHP default Keyword

last modified April 16, 2025

The PHP default keyword serves multiple purposes in different contexts. Primarily used in switch statements as a fallback case, it also appears in array destructuring and function parameter declarations.

Basic Definitions

In switch statements, default specifies code to execute when no case matches. It's optional but provides a safety net for unexpected values.

For arrays, default can specify fallback values during destructuring assignments. This prevents undefined index errors when keys don't exist.

In function parameters, default values make arguments optional. Callers can omit parameters with defaults, which then use the specified value.

Basic switch Statement with default

This example demonstrates the most common use of default in a switch statement.

switch_default.php

<?php

declare(strict_types=1);

$day = "Tuesday";

switch ($day) {
    case "Monday":
        echo "Start of work week";
        break;
    case "Friday":
        echo "Almost weekend!";
        break;
    default:
        echo "Regular weekday";
}

The switch checks the $day variable against specific cases. When no case matches, the default block executes. Default acts as a catch-all for unhandled values. It's conventionally placed last in switch statements.

default in Array Destructuring

This example shows using default values when destructuring associative arrays.

array_default.php

<?php

declare(strict_types=1);

$user = ['name' => 'John', 'age' => 30];

['name' => $name, 'role' => $role = 'guest'] = $user;

echo "Name: $name, Role: $role";

The code extracts array values into variables. The role key doesn't exist, so the default 'guest' value is used. This prevents undefined index errors while providing sensible fallbacks. Defaults work with both indexed and associative arrays.

Function Parameter Defaults

This example demonstrates default parameter values in function declarations.

function_default.php

<?php

declare(strict_types=1);

function greet(string $name, string $greeting = "Hello") {
    echo "$greeting, $name!";
}

greet("Alice");          // Uses default greeting
greet("Bob", "Hi");      // Overrides default

The greet function makes the $greeting parameter optional with a default value. Callers can omit it to use "Hello" or specify a custom greeting. Default parameters must come after required parameters in the declaration.

Multiple Default Cases

This example shows how default works with multiple switch cases.

multiple_default.php

<?php

declare(strict_types=1);

$score = 75;

switch (true) {
    case $score >= 90:
        echo "Grade: A";
        break;
    case $score >= 80:
        echo "Grade: B";
        break;
    case $score >= 70:
        echo "Grade: C";
        break;
    default:
        echo "Grade: F";
}

The switch evaluates each case conditionally. The default case handles scores below 70. Only one case executes due to break statements. This pattern is useful for range-based conditions where if-elseif would be verbose.

Default in match Expression

This example demonstrates default in PHP 8's match expression.

match_default.php

<?php

declare(strict_types=1);

$status = 404;

$message = match($status) {
    200 => 'OK',
    301 => 'Moved Permanently',
    404 => 'Not Found',
    default => 'Unknown Status'
};

echo $message;

The match expression is a more concise alternative to switch. Its default case handles any unlisted status codes. Unlike switch, match does strict comparisons and returns a value. Default is required if not all possible values are covered.

Default Array Merge

This example shows using default values when merging configuration arrays.

merge_default.php

<?php

declare(strict_types=1);

$defaults = ['color' => 'red', 'size' => 'medium'];
$userPrefs = ['color' => 'blue'];

$config = array_merge($defaults, $userPrefs);

print_r($config);

The code merges default values with user preferences. Missing keys in $userPrefs keep their default values. This pattern is common for configuration systems. Array union operator (+) can also be used but behaves differently with numeric keys.

Default Class Properties

This example demonstrates default property values in class definitions.

class_default.php

<?php

declare(strict_types=1);

class Product {
    public string $name = 'Unknown';
    public float $price = 0.0;
    public bool $inStock = false;
}

$product = new Product();
echo "{$product->name}: \${$product->price}";

The Product class defines default values for all properties. New instances start with these defaults unless overridden. Defaults ensure objects always have valid property values. They're particularly useful for data transfer objects and configuration classes.

Best Practices

Switch defaults: Always include a default case in switch statements for robustness.
Array safety: Use default values when destructuring arrays to avoid undefined index errors.
Function parameters: Place parameters with defaults after required ones in function declarations.
Documentation: Clearly document default behaviors in your code comments.
Consistency: Use similar default values across related functions for predictable behavior.

Source

PHP switch Documentation

This tutorial covered the PHP default keyword with examples showing its usage in switch statements, arrays, functions, and class properties.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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