Can't programmatically log user in php

Question

I am looking to programmatically login users from php, but after reviewing multiple threads about achieving this I have found that none of the methods work for me and I am not sure why. Currently I am running the code from the functions.php file of my theme:

function test_login() {

    if (!isset($_GET['uname'])) return;

    function login($user) {
        do_action('wp_login', $user->data->user_login, $user);
        wp_set_current_user( $user->ID, $user->data->user_login );
        wp_set_auth_cookie( $user->ID, true );
        header('hdrs: are_setting');
    }

    $creds = array();
    $creds['user_login'] = $_GET['uname'];
    $creds['user_password'] =  'password';
    $creds['remember'] = true;
    $user = wp_signon( $creds, false );
    if ( is_wp_error($user) ) {
        // echo $user->get_error_message();
    } else {
        login($user);
        wp_redirect( 'https://example.com/home' );
        // exit;
    }
}

add_action('after_setup_theme', 'test_login'); // Also tried init

I have also tried running this code in a separate file, but there is no difference. The code finds the user no problem, the login function is executed and I can even see the hdrs header in my browser. But after the redirect I am still logged out. I do suspect that caching may be involved in this, but I tried disabling all my caching plugins, but there may be something going on with the hosting provider or cloudflare. Either way, a "manual" login works without issue. I hope that I am missing something obvious here.

Answer 1

This modified version of your code works perfectly for me:

function test_login() {
    if ( empty( $_GET['uname'] ) || is_user_logged_in() ) {
        return;
    }

    $user = wp_signon( [
        'user_login'    => sanitize_text_field( wp_unslash( $_GET['uname'] ) ),
        'user_password' => 'password',
        'remember'      => true,
    ] );

    if ( is_wp_error( $user ) ) {
        wp_die( $user->get_error_message() );
    } else {
        wp_clear_auth_cookie(); // clear any user cookies set in the browser
        wp_set_current_user( $user->ID, $user->user_login ); // set current site user
        wp_set_auth_cookie( $user->ID ); // set user cookie
        do_action( 'wp_login', $user->ID, $user ); // trigger core action.
    }
}
add_action( 'after_setup_theme', 'test_login' );