为什么我不能在wordpress中使用$ wpdb

Question

I am trying to make ajax in wordpress.

The php file is placed in plugins folder as a plugin

wp-content/ plugins/ my_plugin/ php-file.php 

I know that Ajax receives informations from the php file because I did some tests, so the url in ajax is correct !

But when I want to pull some data from the database, I get this error in log file:

PHP Fatal error: Uncaught Error: Call to a member function get_results() on null in /home/......

I specify that I have an error 500 in the browser console (network tab)

my code :

global $wpdb;
$table_name = $wpdb->prefix . "wplusersprofiles";
$user = $wpdb->get_results('SELECT * FROM my_table');
$live_Str = $user[0]->etat;   

the problem is therefore is on line 3

Answer 1

Thank you jeff, Yes the 2nd line was not necessary, but according the log the probleme is in 3rd line ...

In fact This code return me data, i can see it in the top of the page , But this data can not be sent via ajax, So i have always error that triggers instead of success :(

This is my js code :

$.ajax({
     url: "https://my-web-site.com/lab1/wp-content/plugins/my-plugin/my-plugin.php",
     type: "POST",
     data: {etat:"verif"},
       success: function(data) {
       
       if (data=='on')
       {
             activate_livestream(); // this is a simple js function
       }
       else {
         console.log('off');
       }

     },
     error:function(){
       alert('ERROOOOR'); // i get always this alert :( !!
     }
   })// $.ajax end 

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