How can PHP developers ensure that the MySQL connection is established before executing query strings to prevent errors?

PHP developers can ensure that the MySQL connection is established before executing query strings by checking if the connection is successful using functions like `mysqli_connect_errno()` or `mysqli_connect_error()`. If the connection is not established, developers can handle the error appropriately, such as displaying an error message or logging the error. This ensures that query strings are only executed when a valid connection to the MySQL database exists.

// Establish MySQL connection
$connection = mysqli_connect($host, $username, $password, $database);

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

// Query strings can now be executed safely
$query = "SELECT * FROM table";
$result = mysqli_query($connection, $query);

// Handle query results as needed