config.inc.php

<?php

$config = [
	'mailbox' => '{ssl0.ovh.net:993/imap/ssl}INBOX',
	'user' => 'user@domain',
	'password' => 'plop',
];

$filters = [
	[   'condition' => fn($headers) => isset($headers->subject) and strpos($headers->subject, "[sysadmin]") === 0 and strpos($headers->subject, "CRITICAL") === false,
		'destination' => 'sysadmin'
	],
	[   'condition' => fn($headers) => isset($headers->subject) and strpos($headers->subject, "[dev]") === 0,
		'destination' => 'dev',
	],
];

imapfilter.php

<?php

require __DIR__.'/cfg/config.inc.php';

$state_file = __DIR__.'/var/state';
if (!file_exists(__DIR__.'/var')) {
	mkdir(__DIR__.'/var');
}

if (!file_exists($state_file)) {
	file_put_contents($state_file, '0');
}

$latest_id = file_get_contents($state_file);

$box = imap_open($config['mailbox'], $config['user'], $config['password']);

$list = imap_sort($box, SORTARRIVAL, 1);

file_put_contents($state_file, $list[0]);

foreach ($list as $id) {
	if ($id <= $latest_id) {
		break;
	}

	filter_message($box, $id);
}

function filter_message($box, $id) {
	global $filters;

	$info = imap_headerinfo($box, $id);
	$info->subject = imap_utf8($info->subject);

	foreach ($filters as $filter) {
		if ($filter['condition']($info)) {
			if (!empty($filter['destination'])) {
				echo "Moving message '{$info->subject}' to '{$filter['destination']}'\n";
				imap_mail_move($box, $id, $filter['destination']);
			}
		}
	}
}