To extend the functionalities of an existing CodeIgniter library, you should create a new library that extends the original library class. This way, you can add or override methods to customize the behavior without modifying the core library file. Extending the library class allows you to reuse existing functionality while introducing your modifications.

The Active Record Class in CodeIgniter handles SQL injection prevention by using parameterized queries. This approach ensures that user inputs are treated as data rather than executable code, making it more difficult for malicious SQL injection attacks to occur. It helps prevent the injection of unauthorized SQL code into database queries, enhancing the security of the application.

Unit testing CodeIgniter applications with heavy database interactions can be challenging due to the need to manage database connections and maintain a clean state between tests. This is crucial to avoid interference between test cases and ensure reliable and accurate results.

The load() method in CodeIgniter is used to load a library, helper, or model. It is a fundamental method for loading various resources into your CodeIgniter application, facilitating code organization and reuse.

The correct sequence for updating a record in CodeIgniter using Active Record is set() to set the values, followed by where() to specify the condition, and then update() to perform the update. update_batch() is used for updating multiple records simultaneously.

When 'strict' mode is enabled, CodeIgniter will automatically roll back a transaction if any database error occurs during its execution. This helps maintain data integrity by preventing the application from continuing with potentially corrupt data. It ensures that transactions are handled more cautiously, and the system responds promptly to any issues that may compromise the transaction.

CodeIgniter's 'Form Validation' class enhances input data integrity by validating and filtering user input, reducing the risk of security vulnerabilities such as SQL injection and XSS attacks. It ensures that only valid and expected data is processed, contributing to overall application security.

In CodeIgniter, the default behavior of transactions is that auto-commit is enabled by default. This means that each SQL statement is treated as a single transaction and is automatically committed.

In a RESTful API built with CodeIgniter, pagination for resource listings is typically implemented by using query parameters such as "page" and "limit" in the API endpoint URL. This allows clients to request specific pages and control the number of items per page.

OAuth in web applications is primarily used for enabling secure third-party access to resources. It allows users to grant limited access to their resources without sharing their credentials.

The key distinction between errors and exceptions lies in their recoverability. Errors are typically non-recoverable issues that result from severe problems, such as out-of-memory errors. On the other hand, exceptions are designed to be caught and handled, allowing for graceful recovery from unexpected situations during runtime.

When integrating a third-party library that requires database interactions, the most crucial CodeIgniter feature is the Database Library. This library provides a set of functions for interacting with the database, making it essential for seamless integration with third-party libraries that involve database operations.