It's in vendor/laravel/framework/src/Illuminate/Foundation/helpers.php:

function redirect($to = null, $status = 302, $headers = [], $secure = null)
{
    if (is_null($to)) {
        return app('redirect');
    }

    return app('redirect')->to($to, $status, $headers, $secure);
}

Because ag ignores vcs ignores(.gitignore, .hgignore; still obey .ignore), now I got what I want: using ag --php --skip-vcs-ignores "function redirect\("

vendor/laravel/framework/src/Illuminate/Foundation/helpers.php
604:    function redirect($to = null, $status = 302, $headers = [], $secure = null)

vendor/symfony/routing/Matcher/RedirectableUrlMatcherInterface.php
30:    public function redirect($path, $route, $scheme = null);

vendor/symfony/routing/Tests/Fixtures/RedirectableUrlMatcher.php
22:    public function redirect($path, $route, $scheme = null)