get_current_user_id() creates blank screen - php

Question

I need to get the current user id to lookup customer information, in order to pass it to an api call to process a payment.
I have tried:
$user = wp_get_current_user();
echo ( isset( $user->ID ) ? (int) $user->ID : 0 );
and:
$userid = get_current_user_id();
echo $userid;
But when I run the php file, the process stops at the first line and does not display anything after it. I dont get an error or nothing, just a blank screen.
I have been searching for solution for 2 weeks. All results says that these codes should work, but does not for me.
I have also added the require('to user.php') and doe not work.
I even enter this php code into Dreamweaver, and "wp_get_current_user" or "get_current_user_id" does not turn blue like other function calls do, but get_current_user() does turn blue, but this is the websites owners user not the current user.
Please help if you can, much appreciated.
Thanks
Here is my code:
<?php
echo "here";
/**
* Lookup the customer information.
*
* #since 1.0.0
*
* #return void
*/
echo "is";
function lookup_customer_information() {
$current_user = wp_get_current_user();
if ( ! is_object( $current_user ) || ! isset( $current_user->ID ) ) {
echo $current_user;
return;
}
echo "the";
echo "code";
// do your business logic here.
}
?>
<?php $userinfo = lookup_customer_information();
echo $userinfo;
?>

Answer 1

From what you describe, it sounds like WordPress Core has not loaded the wp-includes/user.php file yet or the current user has not been setup yet.
WordPress, like all apps, load in a particular order. What may be causing the issue for you is the current user has not been setup and/or the user files (in Core) have not loaded yet.
Let's See if Function is Available
Let's discover if the function wp_get_current_user is ready to be used. You can do any of these:
Check the PHP error logs
Check your settings to display the errors
Put code in to check for it
Let's use the third option to show you if it's available or not. Put this code snippet in above your if conditional you show in your question:
if ( ! function_exists( 'wp_get_current_user' ) ) {
die('The file with wp_get_current_user() has not loaded yet into memory.');
}
The code will use the PHP construct function_exists to check if wp_get_current_user has been loaded into memory. If yes, you will not get the message, as it's ready. Else, the website will stop loading and you'll get the above message displayed on the screen.
No, It's Not Available
If the website renders the above message on the screen, then you know it's not loaded into memory yet. That means you are trying to use it before WordPress Core has loaded it.
How to Fix
Delay. You want to wait until WordPress has setup the user. Typically, you can register to the init event or later. For example, you can do this:
add_action( 'init', 'lookup_customer_information' );
/**
* Lookup the customer information.
*
* #since 1.0.0
*
* #return void
*/
function lookup_customer_information() {
$current_user = wp_get_current_user();
if ( ! is_object( $current_user ) || ! isset( $current_user->ID ) ) {
// you might want to post a message
return;
}
// do your business logic here.
}
Yes, It Is Available
When running the function_exists() snippet above, you did not get the message. Hum, something else is up then.
Now, it's time to dig into your PHP error logs to see what is going on. You can post them to help us help you.