What are some common issues that arise when migrating applications to PHP 5.4?

Issue: Deprecated features such as the MySQL extension and register_globals can cause compatibility problems when migrating applications to PHP 5.4. To resolve these issues, update code to use MySQLi or PDO for database connections and avoid using register_globals.

// Example code snippet for updating database connection from MySQL extension to MySQLi
// Before:
$conn = mysql_connect($host, $username, $password);
mysql_select_db($database, $conn);

// After:
$conn = mysqli_connect($host, $username, $password, $database);