declare(strict_types=1);is code that is inserted at the beginning of a php file and then checks if the php code in the file is strict and valid (if the functions have a return type, if the arguments have values, etc).
More info here: https://www.phptutorial.net/php-tutorial/php-strict_types/
This utility greatly contributes to the correct functioning of the code and prevents a large number of mistyping errors.