This is a pull request that I opened in my own fork to preview the diff and allow code review.

This PR adds a new "Loop" mode that can be used with -L. For example: php -L script.php.

This mode runs the script in a loop.

The big difference with doing a while(true) {} loop in PHP is that every execution of the script is isolated from the other executions.

It's basically a clone of the PHP-FPM (worker) behavior for CLI.

Use cases

• Workers, i.e. processing jobs in a message queue: instead of running in a loop the script can process one job, the "Loop" mode will ensure the script will loop => each job will be processed in a fresh PHP environment, meaning no memory leaks, no shared state, etc.
• AWS Lambda: by running php -L each event/request would be processed in an isolated PHP environment without having the overhead of starting a new process: see Scenario G: custom PHP SAPI bref-bootstrap-benchmarks#14

But wait, we can already do that by keeping the PHP script alive to handle multiple requests/jobs? (like Laravel Octane, Swoole, RoadRunner, etc.)

Yes! It's very similar. Here is a comparison:

Example

Before:

<?php
// script.php
$a = 'foo';
while (true) {
    echo $a . PHP_EOL;
    // the global state can be changed
    // and it will impact next executions
    $a = 'bar';
}

$ php -L script.php
foo
bar
bar
[...]

With the "loop mode":

<?php
// script.php
$a = 'foo';
echo $a . PHP_EOL;
// the global state can be changed
// but it will not impact next executions
$a = 'bar';

$ php -L script.php
foo
foo
foo
[...]

Understanding how it works

This is pseudo code of the new -L mode:

while (is_running) {
        // Boot the PHP engine
	php_request_startup();

        // Execute `index.php` in the current process
        php_run_script("index.php");

        // Reset the PHP engine (clear variables, memory, open resources, etc.)
        php_request_shutdown();
}