php - 如何在 PHP 中使用 MySQL 变量？

Question

我正在尝试做这样的事情：

SET @grade_id = (SELECT  grade_id FROM grades_table WHERE grades = '1' AND sections = 'A');
SET @subject_id = (SELECT subject_id FROM subjects_table WHERE grade_id = @grade_id AND subject_name = 'Maths' AND teacher_id = '1');
INSERT INTO topics_table ( subject_id, grade_id, topic_name) VALUES (@subject_id, @grade_id,'Another Topic'); 

这在 phpmyadmin 中有效，但在 Visual Studio Code 中与 PHP 一起使用时无效。我在 VSC 中这样做：

$topic = "SET @grade_id = (SELECT  grade_id FROM grades_table WHERE grades = '1' AND sections = 'A');
SET @subject_id = (SELECT subject_id FROM subjects_table WHERE grade_id = @grade_id AND subject_name = 'Maths' AND teacher_id = '1');
INSERT INTO topics_table ( subject_id, grade_id, topic_name) VALUES (@subject_id, @grade_id,'Another Topic')"; 

mysqli_multi_query(
    $connection,
    $topic
);
mysqli_close($connection);