我怎么处理mysql_select_db中的$ db变量（“database”，$ db）; [关闭]

Question

What does the $db variable do and what can I do with it after writing it after the first parameter? I have searched for an answer but your answers are almost always better.

<?php 
    mysql_connect("localhost", "username", "password");
    mysql_select_db("database", $db);
?>

I ask this question because I see people using the $db tag a lot with representing the database. They do things like for example this: $db->query("SELECT * FROM questions") or die($db->error);.

Answer 1

$db is usually for database connection. It provides connection link to database, identifies the connection and provides database object. When you use mysql_* functions, you can define last parameter as connection variable. You use it to indentify connection, so multiple mysql connections don't mix up.