What are the differences between PHP processes and threads?

The main differences between PHP processes and threads are as follows:

1. Operation mode: Processes are the smallest unit of resource allocation by the operating system, with each process having its own independent memory space and operating environment, allowing them to run independently. Threads, on the other hand, are the execution units within a process, where multiple threads share the process’s memory space and resources, allowing them to execute concurrently.
2. Resource usage: Processes are independent of each other, each having its own memory space and resources, leading to larger resource usage. On the other hand, threads share the memory space and resources of a process, resulting in relatively smaller resource usage.
3. Switching and communication: Process switching requires saving the current state and context, resulting in a high switching cost. Inter-process communication needs to utilize the process communication mechanisms provided by the operating system. On the other hand, thread switching only requires saving the context, resulting in a lower switching cost. Threads can communicate through shared memory, semaphores, message queues, and other methods.
4. Safety: Processes are relatively secure because errors in one process do not impact others, as they are independent. In contrast, threads share resources within a process, so an error in one thread could potentially cause the entire process to crash.
5. Programming difficulty: Due to the independence of processes, communication and synchronization between processes are more complex, making programming more challenging. On the other hand, threads share resources within a process, making communication and synchronization between threads relatively simple, resulting in a lower programming difficulty.

In summary, processes are suitable for handling independent tasks, while threads are suitable for handling tasks that require sharing resources and concurrent execution. In PHP, it is common to use multiple processes to handle concurrent requests, such as using PHP-FPM to manage multiple PHP processes, rather than using threads.