How can the presence or absence of specific PHP extensions, like mysql, affect the execution of database connection code in PHP?

The presence or absence of specific PHP extensions, like mysql, can affect the execution of database connection code in PHP because these extensions provide the necessary functions to interact with the database. If a required extension is missing, the code will not be able to establish a connection to the database. To solve this issue, you can check for the existence of the required extension before attempting to connect to the database.

// Check if the mysql extension is loaded
if (!extension_loaded('mysql')) {
    die('The mysql extension is not loaded. Please enable it in your PHP configuration.');
}

// Database connection code using mysql extension
$servername = "localhost";
$username = "username";
$password = "password";
$database = "dbname";

$conn = mysql_connect($servername, $username, $password);
if (!$conn) {
    die('Could not connect: ' . mysql_error());
}

mysql_select_db($database, $conn);