yii\web\MultipartFormDataParser::parse PHP Метод

    public function parse($rawBody, $contentType)
    {
        if (!empty($_POST) || !empty($_FILES)) {
            // normal POST request is parsed by PHP automatically
            return $_POST;
        }
        if (empty($rawBody)) {
            return [];
        }
        if (!preg_match('/boundary=(.*)$/is', $contentType, $matches)) {
            return [];
        }
        $boundary = $matches[1];
        $bodyParts = preg_split('/-+' . preg_quote($boundary) . '/s', $rawBody);
        array_pop($bodyParts);
        // last block always has no data, contains boundary ending like `--`
        $bodyParams = [];
        $filesCount = 0;
        foreach ($bodyParts as $bodyPart) {
            if (empty($bodyPart)) {
                continue;
            }
            list($headers, $value) = preg_split("/\\R\\R/", $bodyPart, 2);
            $headers = $this->parseHeaders($headers);
            if (!isset($headers['content-disposition']['name'])) {
                continue;
            }
            if (isset($headers['content-disposition']['filename'])) {
                // file upload:
                if ($filesCount >= $this->getUploadFileMaxCount()) {
                    continue;
                }
                $fileInfo = ['name' => $headers['content-disposition']['filename'], 'type' => ArrayHelper::getValue($headers, 'content-type', 'application/octet-stream'), 'size' => StringHelper::byteLength($value), 'error' => UPLOAD_ERR_OK, 'tmp_name' => null];
                if ($fileInfo['size'] > $this->getUploadFileMaxSize()) {
                    $fileInfo['error'] = UPLOAD_ERR_INI_SIZE;
                } else {
                    $tmpResource = tmpfile();
                    if ($tmpResource === false) {
                        $fileInfo['error'] = UPLOAD_ERR_CANT_WRITE;
                    } else {
                        $tmpResourceMetaData = stream_get_meta_data($tmpResource);
                        $tmpFileName = $tmpResourceMetaData['uri'];
                        if (empty($tmpFileName)) {
                            $fileInfo['error'] = UPLOAD_ERR_CANT_WRITE;
                            @fclose($tmpResource);
                        } else {
                            fwrite($tmpResource, $value);
                            $fileInfo['tmp_name'] = $tmpFileName;
                            $fileInfo['tmp_resource'] = $tmpResource;
                            // save file resource, otherwise it will be deleted
                        }
                    }
                }
                $this->addFile($_FILES, $headers['content-disposition']['name'], $fileInfo);
                $filesCount++;
            } else {
                // regular parameter:
                $this->addValue($bodyParams, $headers['content-disposition']['name'], $value);
            }
        }
        return $bodyParams;
    }