mysqli_insert_id（）总是返回NULL

Question

mysqli_insert_id() always returning null.I cannot understand the reason for this

    $blocksql="insert into blockinfo values(NULL,'$user','$book','$blocktime','$quant1','$flag','$title')";

    mysqli_query($con,$blocksql);
    $id=mysqli_insert_id();

Answer 1

You're not passing a connection to mysqli_insert_id() of course it will always be null.

$id = mysqli_insert_id($con);