Support proc_open(), exec(), passthru(), system()

[adamziel]

Description

Adds support for proc_open(), popen($cmd, "w"), exec(), passthru(), system() via a custom spawn handler defined in JavaScript via php.setSpawnHandler().

proc_open()

This PR patches PHP to replace C calls to proc_open() with a custom implementation provided with a new proc_open7.0.c/proc_open7.4.c file. The original proc_open() relies on POSIX fork() function not available in WebAssembly, so which is why this PR defers the command execution to the setSpawnHandler() callback provided via JavaScript.

exec(), passthru(), system()

This PR provides a custom php_exec implementation (via the wasm_php_exec C function). php_exec is the function powering PHP functions like exec(), passthru(), system().

popen()

The existing popen() implementation is rewired to call the setSpawnHandler() callback. Also, the support for the popen($cmd, "w") mode was added.

Other notes

This PR removes support for PHP 5.6 as supporting these functions was challenging there AND also WordPress dropped the support for PHP 5.6.

Closes #594
Closes #710

[dmsnell]

this seems quite usable, though maybe we need to figure out how to make the difference between browser and Node environments less implicit. if running in Node certain things will work that crash in the browser. I wonder if we can stub the functions in the web version to do something benign.

obviously we can't spawn arbitrary processes because we only have PHP and SQLite built. what is PHP spawning? is this only important when running the WordPress test suites?

[adamziel]

this seems quite usable, though maybe we need to figure out how to make the difference between browser and Node environments less implicit. if running in Node certain things will work that crash in the browser. I wonder if we can stub the functions in the web version to do something benign.

Good call! I added this:

		if (!ENVIRONMENT_IS_NODE) {
			return 1; // proc_open() is not supported in the browser yet.
		}

obviously we can't spawn arbitrary processes because we only have PHP and SQLite built. what is PHP spawning? is this only important when running the WordPress test suites?

Mostly it's spawning php, as in phpunit test runner spawns another process. A follow-up PR would be good to ensure proc_open('php' runs php-wasm as the system installation may be a different PHP version or missing entirely.

Spawning another WebAssembly PHP instance could just work on the web. We could even open up wider proc_open support in the browser by exposing a callback like registerShellCommand() that people could use to shim busybox and other utilities as needed. Let's talk about it once a need arises.