I keep getting a “Call to undefined function..” error displaying - php

Question

I am new to PHP programming and I am trying to teach myself WordPress theme development for fun and I am using PhpStorm as my IDE.
I am trying to better understand the inner-workings of WordPress and I am running into a roadblock on something.
I have a sandbox plugin that I created to use to play around with WordPress.
In my “wp-content/plugins/sandbox/sandbox.php” file, I am just running basic PHP code to help me get used to the language as it relates to WordPress.
Also, I installed both Kint and Whoops using Composer to help with debugging.
Now that I got that out of the way, here is what I am doing:
Code #1
namespace MyDevPlayground\Sandbox;
add_action( 'loop_start', __NAMESPACE__ . '\process_the_string' );
function process_the_string() {
$current_user = wp_get_current_user();
$data_packet = array(
'id' => $current_user->ID,
'email' => $current_user->user_email,
'name' => array(
'first_name' => $current_user->user_firstname,
'last_name' => $current_user->user_lastname,
),
);
render_user_message( $data_packet );
}
function render_user_message( array $current_user ) {
$user_id = $current_user['id'];
d( "Welcome {$current_user['name']['first_name']}, your user id is { {$user_id} }." );
ddd( "Welcome {$current_user['name']['first_name']}, your user id is {$user_id}." );
}
When I run Code #1 above everything is fine and Kint displays the values just fine.
Now for the problem I am having that I don’t understand about WordPress:
Code #2
namespace MyDevPlayground\Sandbox;
add_action( 'loop_start', __NAMESPACE__ . '\check_logged_in_user' );
function check_logged_in_user(){
$current_user = wp_get_current_user();
if ( 0 == $current_user->ID ) {
d('Not logged in');
} else {
ddd('Logged in');
}
}
check_logged_in_user();
When I run Code #2 above, Whoops reports the following error:
Call to undefined function MyDevPlaygroundSandbox\wp_get_current_user
For some reason when I run Code #1, the wp_get_current_user() function loads just fine, but not with Code #2.
Can someone help me understand why this is in laymen’s terms if possible?
What is the difference between Code #1 and Code #2?
How come the wp_get_current_user() function is not loading in Code #2, but it is in Code #1?
Thank you for your help.

Answer 1

when you use "add_action" command you can not use function name for calling that action,
you need to use the call command like this :
do_action("check_logged_in_user");
more information : https://developer.wordpress.org/reference/functions/add_action/