Variable template in mustache style

mustache-vars.php

<?php

function mustache(array $context) {
    return function($resource) use($context) {
        $handler = resource_of($resource);
        $state = @out;
        $tag = '';

        while (!feof($handler)) {
            $out = fgets($handler);
            $len = strlen($out);
            $pos = 0;

            for ($i = $pos; $i < $len; $i++) {
                if ($state === @out && $out[$i] === '{' && ($out[$i+1] ?? '') === '{') {
                    yield substr($out, $pos, $i - $pos);
                    $pos = $i + 2;
                    $tag = '';
                    $state = @tag;
                } elseif ($state === @tag && $out[$i] === '}' && ($out[$i+1] ?? '') === '}') {
                    $tag = trim($tag . substr($out, $pos, $i - $pos));
                    $pos = $i + 2;
                    $state = @out;

                    switch ($tag[0]) {
                        default:
                            if (!isset($context[$tag])) {
                                throw new \RuntimeException("Variable \"$tag\" not found");
                            }
                            yield $context[$tag]; // TODO escape
                    }
                }
            }

            if ($state === @out) {
                yield substr($out, $pos);
            } elseif ($state === @tag) {
                $tag .= substr($out, $pos);
            }
        }

        if ($state === @tag) {
            throw new \LogicException('Unbalanced tags');
        }

        fclose($handler);
    };
}