The short answer is no. Laravel 13 wants PHP 8.3 or 8.4. Laravel 8’s ceiling was 8.1. Those ranges are not marketing; they are what composer.json of laravel/framework will allow, and what Illuminate will actually run without using syntax your interpreter does not have.
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Upgrade PHP first, or Laravel first?
If you are on Laravel 10 and PHP 8.1, you can move to PHP 8.2 while staying on 10, because 10 spans 8.1–8.3. You cannot jump to Laravel 13 until PHP is 8.3+. If you are on Laravel 8 and PHP 8.3 already, you are off the Laravel 8 chart; the framework is the thing that must move, not PHP. The Laravel PHP version checker is a static matrix for 8 through 13 so you can pick both numbers and see the official band without opening six upgrade guides.
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Platform packages on this site follow the same rule: the app is Laravel 13 on 8.3/8.4. A package that still requires ^8.0 of the framework will not install, which is a Composer problem more than a PHP problem. That is the other half of the story, and it belongs in caret versus tilde constraints.
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What breaks when you ignore the band
Typed constants, json_validate(), and newer Carbon/PHPUnit combinations show up as parse errors or as Composer conflicts, not as a friendly banner. Test the matrix before you change platform php in composer.json. After the interpreters match, generate a fresh APP_KEY only for a new environment, not as a side effect of the upgrade. Rotating the key is a separate decision.
Write the allowed PHP range in the README of each app you still maintain. Future you will not remember whether that 10.x box was allowed to go to 8.3.
