A “Call to undefined function..” error keeps displaying

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?

Thank you for your help.