I found it in the mysql help document when searching for mysql_pconnect, and recorded it here.
The web server uses PHP to generate a web page in the following three ways:
1: Use PHP as a CGI Wrapper. In this way, each request to the web server will cause a php parser process to be created. When the execution of the php page ends, the php parser process will terminate.
Two: In a multi-process web server, use php as a module of the web server. This is currently the most popular approach, but it is currently only used by apahce. A multi-process web server usually has a parent process that coordinates the work of a bunch of child processes. When a page request arrives at the web server, the parent process hands the request to an idle child process for completion. Two requests from the same client are not necessarily processed by the same web server's child process.
Three: In a multi-threaded web server, use php as a plug-in. For example, IIS uses this method. It works the same as multi-processing, but it uses threads instead of processes.