Codeigniter version 1.0 to current

CI 1.6 worked on PHP 5.2.

According to Google Gemini:

PHP Version for CodeIgniter 1.0

CodeIgniter 1.0 is compatible with the following PHP versions:

    PHP 4.3.0 or higher
    PHP 5.2.0 or higher (recommended)

Please note:

    Some features may not work with PHP 5.3.0 and later.
    CodeIgniter 1.0 was released in October 2012 and reached end-of-life support in June 2014.
    Using older PHP versions is strongly discouraged due to potential security vulnerabilities.
    We highly recommend using the latest version of PHP.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper