这两个变量$name 和$email 函数内部未知。您必须通过更改global $wpdb 进入global $wpdb, $name, $email:

require_once(\'../../../wp-load.php\');

/**
 * After t f\'s comment about putting global before the variable.
 * Not necessary (http://php.net/manual/en/language.variables.scope.php)
 */
global $name = $_POST[\'name\'];
global $email = $_POST[\'email\'];

function insertuser(){
  global $wpdb, $name, $email;
  $table_name = $wpdb->prefix . "newsletter";
  $wpdb->insert($table_name, array(\'name\' => $name, \'email\' => $email) ); 
}

insertuser();

require_once(\'../../../wp-load.php\');

$name = $_POST[\'name\'];
$email = $_POST[\'email\']

function insertuser( $name, $email ) {
  global $wpdb;

  $table_name = $wpdb->prefix . \'newsletter\';
  $wpdb->insert( $table_name, array(
    \'name\' => $name,
    \'email\' => $email
  ) );
}

insertuser( $name, $email );

require_once(\'../../../wp-load.php\');

global $wpdb;

$name = $_POST[\'name\'];
$email = $_POST[\'email\'];
$table_name = $wpdb->prefix . "newsletter";
$wpdb->insert( $table_name, array(
    \'name\' => $name,
    \'email\' => $email
) );