What does it mean when php selects the db of the database?

When using PHP to develop web applications, you often need to interact with the database, and selecting a database is the first step in establishing a database connection. What does "db" mean?

DB is the abbreviation of Database, which stands for "database". When we use PHP to write a program to connect to a database, we need to specify the name of the database. This name is what we usually call "db". In different operating systems and databases, "db" may be written differently. For example, in MySQL, "db" needs to be preceded by a "mysql_" prefix, that is, "mysql_db".

Specifically, when using PHP to connect to the MySQL database, we need to use the "mysqli_connect()" function to connect. This function accepts four parameters, namely host name, user name, password and database name. Among them, the fourth parameter is the "db" we need to specify.

For example, the following code shows how to connect to a MySQL database using PHP and select a database named "example_db":

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "example_db";

// 创建连接
$conn = mysqli_connect($servername, $username, $password, $dbname);

// 检测连接
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
echo "Connected successfully";
?>

In the above code, we use "mysqli_connect( )" function creates a connection object named "$conn" and passes four parameters, namely "localhost" (indicating the local host), "root" (MySQL's default administrator account), and empty string ( means no password) and "example_db" (the database we need to connect to).

If the connection is successful, the program will output the "Connected successfully" statement, otherwise it will output a connection error message.

In short, the "db" of the selected database refers to the name of the database we need to connect to. It is one of the necessary steps to establish a database connection.

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