如何用PHP重写下面这段java的Sha256WithRSA生成签名？

如何用PHP重写下面这段java的Sha256WithRSA生成签名？

     /**
     * 签名
     *
     * @param: plain，明文
     * @param: pkcs8 私钥
     */
    public static byte[] sign(byte[] plain, String priKey) {
        try {
            PrivateKey privateKey = loadPriKey(priKey);
            Signature signature = Signature.getInstance("Sha256WithRSA");
            signature.initSign(privateKey);
            signature.update(plain);
            return signature.sign();
        } catch (Throwable t) {
            throw new RuntimeException(t);
        }
    }
 
 
    /**
     * 加载私钥数据
     *
     * @param: pubKey，私钥文本
     */
    public static PrivateKey loadPriKey(String priKey) {
        try {
            //1.读取私钥文件,获取私钥数据
            byte[] bytesPrivateBase64 = readPriKey(priKey);
            //2.对读取回来的数据进行Base64解码
            byte[] bytesPrivate = Base64.getDecoder().decode(bytesPrivateBase64);
            //3.把解码后的数据,重新封装成一个PrivateKey对象
            PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(bytesPrivate);
            return KeyFactory.getInstance("RSA").generatePrivate(keySpec);
        } catch (Throwable t) {
            throw new RuntimeException(t);
        }
    }
 
    private static byte[] readPriKey(String key) {
        try {
            BufferedReader bufferedReader = new BufferedReader(new StringReader(key));
            String str = null;
            StringBuilder stringBuilder = new StringBuilder();
            while ((str = bufferedReader.readLine()) != null) {
                if (str.contains("---")) {
                    continue;
                }
                stringBuilder.append(str);
            }
            return stringBuilder.toString().getBytes();
        } catch (Throwable throwable) {
            throw new RuntimeException(throwable);
        }
    }