Nginx get / from /index.php without "directory index is forbidden" elsewhere

I need / to be shown like /index.php.

Right now I have

index index.php;
if ( $request_uri = /index.php ) { return 301 /; }
location / { try_files $uri $uri/ =404; }

And everything works fine but there are some errors like

directory index of "/home/.../public_html/tmp/" is forbidden

which I want to remove. The problem here is $uri/ in try_files $uri $uri/ =404;

https://stackoverflow.com/questions/19285355/nginx-403-error-directory-index-of-folder-is-forbidden/38046124#38046124

Ubuntu + Nginx: directory index of "/var/www/app/my-app/" is forbidden

But if I change this to try_files $uri =404; or try_files $uri /index.html index.php =404; then "/" just stops working and shows 404

Why does removing $uri/ prevent showing index.php if I have index index.php;?

How can I set that / should check index.php and if there is no index.php then just shows 404 without throwing "is forbidden" error?

The $uri/ term in a try_files statement has two distinct purposes.

If the URI does not end with a /, but does reference a directory, Nginx will generate a 301 redirection to append a /.

If the URI ends with a / and references a directory, the Index functionality will be invoked, which can either result in an internal redirection to an index file, or a 403 response if the index file is not found.

One solution to suppressing the 403 response without affecting too many other things is to handle URIs which end with a / differently.

For example:

location ~ /$ { 
    if (-f ${request_filename}index.php) {
        rewrite ^(.*)/$ $1/index.php last;
    }
    return 404;
}

The above will test for the existence of index.php within the directory, and return 404 if it does not exist.

The following approach might work:

location = /index.php {
    return 301 /;
}

location / {
    try_files $uri $uri/ /index.php =404;
}