What are common reasons for the "Error: No database selected" message in PHP scripts?

The "Error: No database selected" message in PHP scripts typically occurs when a database connection is established but no specific database is selected for use. To solve this issue, you need to explicitly select the database you want to work with after establishing the connection in your PHP script.

// Establish database connection
$conn = mysqli_connect("localhost", "username", "password");

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

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