We can think of sequential and parallel programming as counterparts.
In sequential programming, we're executing processes in order. This means that a process is started when the preceding process has finished. In other words, there is always only one process being executed. The following figure illustrates this principle:
In parallel programming, multiple processes can be executed concurrently:
To make this easier to understand and more relevant to PHP, we can, instead of processes, think of lines of code. PHP interpreter is always sequential and it never executes code in parallel.
In Chapter 9, Multithreaded and Distributed Computing with pthreads and Gearman, we'll use PHP module pthreads that makes it possible to run PHP code in multiple threads, but we'll see that it's not as simple as it seems. Module pthreads, in fact, creates multiple independent PHP interpreters, each running in a separate thread.