The default PHP script execution mode defines how PHP code will be processed in new virtual servers.

Disabled mode means that PHP scripts are not executed by the server but are instead treated as plain text files. This mode can be used when PHP processing is not required or when serving PHP code as downloadable files.

FPM is the preferred mode, offering high efficiency and advanced features like adaptive process management. It operates as a standalone FastCGI server, ideal for high-traffic sites due to its robust performance capabilities.

FCGI is useful for servers where FPM is not available. It also supports per-virtual-host configurations, though it generally doesn't match FPM in terms of scalability and management features.

CGI mode runs each PHP script as a separate process through the CGI protocol. This mode is less efficient than FPM or FCGI due to the overhead of creating a new process for each request. It's rarely used in modern setups due to its performance limitations.

mod_php integrates PHP directly into the Apache web server, allowing PHP scripts to execute within the server's own process. However, due to security and isolation issues, plus the discontinuation of support and updates, it is now obsolete.