Fix PHP 8.4 deprecation: Implicitly marking parameter as nullable is deprecated, the explicit nullable type must be used instead

PHP 8.4 will be released in November 2024, but the list of new features and deprecated features is already very established. It is clearly detailed on php.watch/versions/8.4.

The most prominent change is the deprecation of implicit nullable parameter declarations:

function test(string $test = null) {}

test('PHP'); // Allowed
test(null); // Allowed

which is easy to fix using the explicit nullable type introduced in PHP 7.1:

- function test(string $test = null) {}
+ function test(?string $test = null) {}

You can detect this depreciation in your PHP files using PHP's built-in linter. This requires a local installation of PHP 8.4.

php -l src/**/*.php

Now that you know the affected files, you can fix them by yourself, or automatically using PHP-CS-Fixer or Rector.

Fix your codebase using PHP-CS-Fixer

PHP Coding Standards Fixer is a tool to automatically apply formatting rules on your code. In our case, we will use only the nullable_type_declaration_for_default_null_value rule.

1 - Install php-cs-fixer:

composer require --dev friendsofphp/php-cs-fixer

2 - Copy the minimal configuration in .php-cs-fixer.php, at the root of your project. We assume your source files are located in src but you can change it to set the path of all your PHP files.

<?php

$finder = PhpCsFixer\Finder::create()
    ->in([__DIR__ . '/src'])
    ->name('*.php')

return (new PhpCsFixer\Config())
    ->setFinder($finder)
    ->setRules([
        'nullable_type_declaration_for_default_null_value' => true,
    ]);

3 - Run php-cs-fixer

vendor/bin/php-cs-fixer fix

Fix your codebase using Rector

Rector is an excellent tool for refactoring and modernizing code. In our case, we will only use the ExplicitNullableParamTypeRector rule.

1 - Install rector:

composer require --dev rector/rector

2 - Copy this minimal configuration in rector.php, at the root of your project. We assume your source files are located in src but you can change it to set the path of all your PHP files.

<?php

return Rector\Config\RectorConfig::configure()
    ->withPaths([__DIR__ . '/src'])
    ->withPhpVersion(Rector\ValueObject\PhpVersion::PHP_84)
    ->withRules([
         Rector\Php84\Rector\Param\ExplicitNullableParamTypeRector::class,
    ]);

3 - Run rector:

vendor/bin/rector

I hope this quick tutorial has saved you some time. There are hundreds of projects to patch on GitHub, have fun!