How to pass variable from wp_loaded hook to custom function?

passing hook $_GET value from hook to function.

The reason I’m using wp_loaded hook is because he is the only one that able to receive $_GET values.

This is not a thing.

Hooks don’t receive $_GET values, that’s not something that happens or needs to be done. You can use $_GET anywhere.

You can safely remove the add_action call, it does not help, and does not do anything. It does not need to be replaced with anything. As I mentioned before, you can use $_GET everywhere. There are no restrictions imposed by WordPress on where it can be used, and you don’t need to “recieve it” to use it, it’s already there.

This was all you ever needed:

$sender = getsender();

More accurately:

$sender = $_GET['sender'];

It’s much more likely that you used method="post" on a form or used POST for AJAX, or that there’s been a mistake somewhere else in the code.

For example in getsender there’s a major mistake:

function getsender() {
    if ( isset( $_GET['sender'] ) ) {
        $sender = $_GET['sender'];
        return $sender;
    }
    // BUT WHAT HAPPENS HERE?!
}

If $_GET['sender'] doesn’t exist then the if statement never happens, and nothing is returned, so it’ll get null or an empty value back and you’ll never know why. It needs to return a value, even if it’s a default value or an error message/value to check for.

but the add_action hook always return true.

This is a misunderstanding of what add_action does. It’s returning true because you told it to run the function getsender when the wp_loaded event/hook happens, and it’s returning true for success that it encountered no problems noting that down. This does not mean that it immediatley executed your function, or that it will return the functions value, that’s not what add_action is for. Think of it like scheduling a meeting in a calendar.

add_action lets you tell WP to call a function/callback when an event/hook happens with a particular name. It’s useful for controlling when code runs and doing things on certain events. E.g. you could run code after a post gets saved using the save_post action.

It has nothing to do with retrieving variables.