CodeIgniter 4 requires a minimum PHP version of 7.2. It utilizes features and improvements introduced in PHP 7.2 and later, ensuring compatibility and taking advantage of the latest language enhancements.

In CodeIgniter, when adding a new table to the database without disrupting existing data, developers should create a migration. Migrations allow for version control of the database schema, enabling easy management of changes while preserving data integrity.

The 'third_party' directory is intended for external libraries and packages, while the 'libraries' directory is meant for custom libraries created within the application.

CodeIgniter suggests using the CIUnitTestCase class and the DBUnitTestCaseTrait trait to simulate a database in unit tests. This helps in testing database interactions without affecting the actual database.

The recommended method for sending form data to a view in CodeIgniter is by loading the form helper using $this->load->helper('form'). This helper provides functions to create form elements and handle form submissions efficiently.

The correct function for retrieving the final query string in CodeIgniter's Query Builder is last_query(). It is a valuable tool for debugging, as it allows developers to inspect the actual SQL query that was generated by the Query Builder.

CodeIgniter leverages its ORM feature to manage complex data relationships in RESTful APIs efficiently. The ORM simplifies database interactions, allowing developers to define and work with relationships between different resources. This enhances the maintainability and readability of the code, making it easier to handle complex data scenarios in a RESTful API context.

In a CodeIgniter API converting database query results to XML and JSON, the most critical aspect for performance optimization is optimizing the underlying database queries. Efficient queries contribute significantly to overall API response time.

CodeIgniter typically manages custom error pages in the application/views/errors directory. This is where you can create custom views for various error types, providing a more user-friendly experience for your application's users when errors occur.

The subject() function in the Email Class is essential when sending an email. It is used to set the email subject, providing a clear and concise description of the email's content. This parameter is crucial for conveying the purpose or topic of the email to the recipient.

CodeIgniter's Hooks feature allows the extension of custom libraries by providing points in the execution flow where additional functionality can be added.

DomainKeys Identified Mail (DKIM) is a method used to verify the authenticity of the sender's identity in email communication. It involves adding a digital signature to the email message, which can be verified by the recipient's email server. This helps prevent email spoofing and ensures the integrity of the email content.