用于学习目的，为什么PHP会对以下代码发出警告:
Warning: mysqli_stmt::close() [mysqli-stmt.close]: invalid object or resource mysqli_stmt in C:'public_html'admin'disctrack.php on line 14
12 $conn = connect('w'); // connect to a database in write mode
13 $stmt = $conn->stmt_init(); // initialize a prepared statement
14 $stmt->close(); // close statement
来自mysqli::stmt_init文档:
在mysqli_stmt_prepare()被调用之前，任何对mysqli_stmt函数的后续调用都将失败。