Running the same script at the same time and PHP's single-threaded status

Question

I know PHP is single-threaded, so I am wondering, does this mean if I run say foo.php in a browser tab and then open another browser tab and run the same file, will it still run if foo.php is still running in the other browser tab!?

If not, how can you run the same script at once? Do you just rename the script something else and then you can run both at the same time? ...or am I getting confused here?

Answer 1

PHP is NOT single threaded by nature.

http://php.net/Thread

It is, however, the case that the most common installation of PHP on unix systems is a single threaded setup, as is the most common Apache installation, and nginx doesn't have a thread based architecture whatever.

In this most common unix setup, each http server process accepting requests can do one of a few things:

• spawn a php interpreter process (cgi - generic CGI gateway)
• manipulate an in process instance of the interpreter (mod_php5 - apache specific PHP module, services both mpm and worker installations of Apache)
• communicate with an FCGI process (php-fpm - commonly used with nginx, some apache usage goes on still ...)

In the most common Windows setup and some more advanced unix setups, PHP can and does operate multiple interpreter threads in one process.

In any case, each instance of the interpreter, be it in another process or in another thread, has access to a unique representation of the script that is a result of compiling the PHP script into opcodes for execution by Zend.

This is by design, it's referred to as a shared nothing architecture and it is the thing allowing you to create hundreds/thousands/millions of instances of the same script that for the most part do not interfere with each other.

Even where PHP utilizes opcode caching to save resources on the compilation stage of execution, no two instances of the interpreter access the same physical memory.

Noteworthy: PHP is not completely void of the effects of concurrent execution in any setup, this is the nature of modern computing; for example if your script edits a text file, and 1000 clients come at once, what do you think will happen to the text file ?