ReadOnlyFileStreamWrapper.php

<?php

class ReadOnlyFileStreamWrapper{
	const SCHEME_RAW = "pmmpreadonly";
	const SCHEME = self::SCHEME_RAW . "://";

	private $realPointer;

	public function stream_open($path, $mode){
		assert(substr($path, 0, strlen(self::SCHEME)) === self::SCHEME);
		$this->realPointer = fopen(substr($path, strlen(self::SCHEME)), $mode);
		stream_set_read_buffer($this->realPointer, 1024 * 16); //16KB
	}

	public function stream_read($count){
		return fread($this->realPointer, $count);
	}

	public function stream_close(){
		fclose($this->realPointer);
	}

	public function stream_seek($offset, $whence = SEEK_SET){
		return fseek($this->realPointer, $offset, $whence);
	}
	
	public function stream_tell(){
		return ftell($this->realPointer);
	}
	
	public function stream_eof(){
		return feof($this->realPointer);
	}

	public function stream_truncate($size){
		return ftruncate($this->realPointer, $size);
	}
}

ReadOnlyMcRegion.php

<?php

class ReadOnlyMcRegion extends \pocketmine\level\format\io\region\McRegion{
	protected function loadRegion(int $x, int $z){
		if(!isset($this->regions[$index = Level::chunkHash($x, $z)])){
			$this->regions[$index] = new ReadOnlyRegionLoader($this, $x, $z, static::REGION_FILE_EXTENSION);
		}
	}
}

ReadOnlyRegionLoader.php

<?php

use pocketmine\level\format\io\region\{RegionLoader, McRegion};

stream_wrapper_register(ReadOnlyFileStreamWrapper::SCHEME_RAW, ReadOnlyFileStreamWrapper::class);
class ReadOnlyRegionLoader extends RegionLoader{
	public function __construct(McRegion $level, int $regionX, int $regionZ, string $fileExtension = McRegion::REGION_FILE_EXTENSION){
		parent::__construct($level, $regionX, $regionZ, $fileExtension);
		fclose($this->filePointer);
		$this->filePointer = fopen(ReadOnlyFileStreamWrapper::SCHEME . $this->filePath, "rb");
	}
}