如果未指定mysql链接，mysql函数是否会打开一个新的mysql连接？

Question

Do functions like mysql_query, mysql_real_escape or mysql_error open a new mysql link if it's not specified ?

php.net says that

The MySQL connection. If the link identifier is not specified, the last link opened by mysql_connect() is assumed

But what if the connection was made inside a class ? Does php know how to find that link ?

(I'm using the codeigniter framework if that's of any help)

Answer 1

What you are actually asking is if your database connection will be made in respect to the Singleton Pattern or not, and the answer is obviously "no". If you instantiate the class in multiple places, each instance will have its own resource. As a consequence, what this mean is that regardless of the instance from which you call mysql_query, it will use the last instance that will be detected, which is actually created during your last instantiation of the class that makes the connection.

[Later edit]

You will get into that situation if you have something similar with:

    //Database.class.php

    class Database {
        public function connect_db() {
             //connection logic here
        }

        public static function query_db($query) {
             $this->connect_db();
             //query database logic here
        }

    }


    //index.php
    include "path/to/Database.class.php";

    $sql = "SELECT * FROM `whatever_i_want`";
    Database::query_db($sql);

    //repeat the last two lines multiple times

Now, this is bad design. In this scenario you will instantiate a connection each time you query the database. To avoid this, the following small change should do the trick:

    //Database.class.php
    public function __construct() {
         $this->connect_db();
    }

    public function query_db($query) {
          //query database logic here
          //note the missing call to connect_db()
    }
    //the rest remains unchanged

    //index.php
    $db = new Database();

    $db->query_db($sql);
    //the rest remains unchanged

Please bear in mind that this is not Singleton, the pattern I previously told you about, and it's not good design either. This is just for the purpose of showing how PHP decides which connection to use. The last example makes the database connection only once, and after that, no matter how many queries you run, it'll use the connection resource got from instantiating the class.

Now, if we modify the last example and we add this to index.php:

    //index.php
    $db2 = new Database();

    $db2->query_db($sql);
    $db->query_db($sql);

Both queries will use the resource got from creating the object $db2, since that will be the last one that PHP will detect.