Default Argument Values

Give arguments a fallback value used when none is supplied.

Syntaxfunction name($arg = defaultValue) { ... }

You can assign a default value to an argument. If the caller does not provide that argument, the default is used.

Arguments with defaults should come after arguments without defaults.

PHP 8 also supports named arguments, letting you pass values by name in any order.

Example

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When to use it

Give a pagination function a default $perPage of 20 so callers can omit it for standard listings.
Default a $format argument to 'json' in an API response helper so most callers don't need to specify it.
Set a default $separator of ', ' in an array-join utility so it works sensibly without arguments.

More examples

Simple default argument

Default values are used when the caller omits that argument; required parameters must come before optional ones.

<?php
function paginate(int $page = 1, int $perPage = 20): array {
    $offset = ($page - 1) * $perPage;
    return ['offset' => $offset, 'limit' => $perPage];
}

print_r(paginate());        // offset:0, limit:20
print_r(paginate(3, 10));   // offset:20, limit:10

Default null for optional objects

Defaulting a parameter to null signals it is optional; ?? inside the function provides a usable fallback.

<?php
function render(string $template, ?array $data = null): string {
    $data ??= [];
    ob_start();
    extract($data);
    include $template;
    return ob_get_clean();
}

Default array and string values

Scalar defaults give a sensible out-of-the-box experience while letting callers override any combination.

<?php
function listItems(array $items, string $sep = ', ', string $wrapper = ''): string {
    $joined = implode($sep, $items);
    return $wrapper ? "$wrapper$joined$wrapper" : $joined;
}

echo listItems(['a', 'b', 'c']);            // a, b, c
echo listItems(['a', 'b', 'c'], ' | ', '*'); // *a | b | c*

Discussion

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