Using the right script execution mode is an essential step in optimizing the performance, scalability, and efficiency of your PHP web applications. The choice depends on your specific needs, server environment, and traffic expectations.

This guide explores the three primary execution modes—FPM (FastCGI Process Manager), FCGI (FastCGI), and CGI (Common Gateway Interface)—explaining their features, advantages, and recommended use cases.

1. FPM (FastCGI Process Manager)

FPM is an advanced implementation of the FastCGI protocol, specifically designed for high-performance environments with demanding workloads. It offers robust process management capabilities, allowing for precise tuning of worker processes, including options for starting, maximum, and idle processes.

This mode is particularly well-suited for modern, high-traffic websites because of its ability to handle heavy loads efficiently. It also supports the use of PHP pools, which can isolate applications or users, enhancing both security and resource allocation.

FPM provides several management modes for its worker processes. These modes determine how the manager will create, maintain, and recycle the child processes that handle PHP requests.

Advanced features like adaptive process spawning and per-pool settings provide additional flexibility for managing server resources effectively. Overall, FPM is the go-to choice for applications requiring scalability, performance, and reliability.

Dynamic FPM

In the dynamic mode, PHP-FPM dynamically adjusts the number of spawned child processes based on the demand. Benefits include automatic adjustment of the number of child processes based on demand, ensuring optimal resource utilization. This is especially useful for servers with fluctuating loads.

Static FPM

In static mode, PHP-FPM maintains a fixed number of child processes. Benefits include consistent memory usage due to a constant number of child processes. This is ideal for servers with stable loads.

On Demand FPM

In the on demand mode, no children are created at startup. Instead PHP-FPM will spawn child processes only when needed, i.e., when there’s an incoming request that needs processing.

2. FCGI (FastCGI)

FCGI builds upon the traditional CGI model by addressing its performance limitations. Unlike CGI, which creates a new process for each incoming request, FCGI keeps processes alive to handle multiple requests, significantly reducing the overhead of process creation and destruction.

While simpler than FPM, FCGI strikes a balance between improved performance and ease of implementation, making it a practical choice for websites that need better performance than CGI but do not demand the advanced features provided by FPM. Its moderate scalability and resource efficiency make it a suitable option for mid-level traffic applications.

3. CGI (Common Gateway Interface)

As one of the earliest methods of executing server-side scripts, CGI operates by creating a new process for each request. While this approach ensures strong security through process isolation, it comes with a significant drawback: high resource consumption.

CGI’s simplicity, however, makes it an attractive option for beginners and for applications with minimal traffic. Its straightforward setup process makes it ideal for basic use cases where ease of use outweighs the need for performance optimization.

Comparative Overview

Feature	FPM	FCGI	CGI
Performance	High	Medium	Low
Resource Efficiency	Excellent	Good	Poor
Scalability	Highly Scalable	Moderate	Poor
Ease of Setup	Moderate	Moderate	Easy
Security	High (with pools)	Moderate	High

Recommendations

For modern applications requiring scalability and top-tier performance, FPM is the most suitable option. If you need better performance than CGI but can forgo FPM’s advanced features, FCGI offers a balanced alternative. Finally, CGI is best reserved for simple, low-traffic setups where ease of setup and use is the primary concern.