The mysqli_select_db() function in PHP is used to select a specific database to work with after a connection to the MySQL server has been established using mysqli_connect().

🔹 Syntax

mysqli_select_db(connection, database);

• connection: A mysqli object returned by mysqli_connect().
• database: Name of the database to select.

✅ Example 1: Basic Usage

<?php
// Connect to MySQL server
$conn = mysqli_connect("localhost", "root", "", "");

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Select database
if (mysqli_select_db($conn, "my_database")) {
    echo "Database selected successfully.";
} else {
    echo "Error selecting database: " . mysqli_error($conn);
}

mysqli_close($conn);
?>

✅ Example 2: Select Database After Connecting Without One

You can connect without specifying the DB name initially, and then select it using mysqli_select_db().

$conn = mysqli_connect("localhost", "username", "password");

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Select the database later
mysqli_select_db($conn, "my_database");

🧠 Note

• If the database is not found, mysqli_select_db() returns false.
• This function is typically used when you want to switch databases after the connection or when the database name is not known at the time of connection.

✅ Alternative (in constructor)

You can also specify the database while connecting:

$conn = mysqli_connect("localhost", "username", "password", "my_database");

In this case, using mysqli_select_db() is not necessary unless you want to change the database afterward.