How to implement Alipay payment in PHP

/**
 * 将要参与签名的参数按要求拼接
 * @param $data
 * author 江南极客
 * @return string
 */
function signQueryString($data){
    // 去空
    $data = array_filter($data);
    //签名步骤一：按字典序排序参数
    ksort($data);
    $string_a = http_build_query($data);
    $string_a = urldecode($string_a);
    return $string_a;
}
 
/**
 * 支付宝RSA签名加密
 * @param $data  要参与加密的参数
 * @param $private_key  应用私钥
 * author 江南极客
 * @return array|string
 */
function RSASign($data,$private_key){
    //要签名的参数字符串
    $query_string = signQueryString($data);
    //应用私钥
    $private_key = chunk_split($private_key, 64, "\n");
    $private_key = "-----BEGIN RSA PRIVATE KEY-----\n$private_key-----END RSA PRIVATE KEY-----\n";
    $private_key_id = openssl_pkey_get_private($private_key);
    if ($private_key_id === false){
        return array(-1,'提供的私钥格式不对');
    }
    $rsa_sign = false;
    if($data['sign_type'] == 'RSA'){
        $rsa_sign = openssl_sign($query_string, $sign, $private_key_id,OPENSSL_ALGO_SHA1);
    }else if($data['sign_type'] == 'RSA2'){
        $rsa_sign = openssl_sign($query_string, $sign, $private_key_id,OPENSSL_ALGO_SHA256);
    }
    //释放资源
    openssl_free_key($private_key_id);
    if ($rsa_sign === false){
        return array(-1,'签名失败');
    }
    $signature = base64_encode($sign);
    return $signature;
}
 
/**
 * 支付宝支付
 * @param array $params  构造好的支付参数
 * author 江南极客
 * @return array|string
 */
function aliPay(array $params){
    $public = [
        'app_id' => $params['app_id'],
        'method' => $params['method'],
        'sign_type' => $params['sign_type'],
        'format' => 'JSON',
        'charset' => 'utf-8',
        'version' => '1.0',
        'timestamp' => date('Y-m-d H:i:s'),
        'biz_content' => $params['biz_content'],
    ];
    if(!empty($params['notify_url'])){
        $public['notify_url'] = $params['notify_url'];
    }
    if(!empty($params['return_url'])){
        $public['return_url'] = $params['return_url'];
    }
    $sign = RSASign($public,$params['private_key']);
    if(is_array($sign)){
        return $sign;
    }
    $public['sign'] = $sign;
    $url = 'https://mapi.alipay.com/gateway.do?'. http_build_query($public,'', '&');
    return $url;
}

$biz_content = [
    'body' => '测试商品x1',
    'subject' => '测试商品',
    'out_trade_no' => date('YmdHis').rand(1000,9999),
    'product_code' => 'QUICK_WAP_WAY',
    'total_amount' => 0.01,
];
$notify_url = "https://xxxxxxxx/notify.php";//通知回调地址(必须是可以无障碍访问没有登录验证的地址)
$params = [
    'app_id'  => '2017xxxxxxxxx6554',//appid
    'method'  => 'alipay.trade.wap.pay',//接口名称
    'sign_type'  => 'RSA2',//签名加密方式
    'notify_url'  => $notify_url,
    'biz_content'  => json_encode($biz_content),//请求参数
];
$params['private_key'] = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";//应用私钥
$data = aliPay($params);
print_r($data);

/**
 * 支付宝验证签名
 * @param $return_data  支付宝服务器推送给notify_url的数据
 * @param $public_key 支付宝公钥
 * author 江南极客
 * @return bool|int
 */
function RSAVerify($return_data, $public_key){
    if(empty($return_data) || !is_array($return_data)){
        return false;
    }
    //支付宝公钥
    $public_key = wordwrap($public_key, 64, "\n", true);
    $public_key = "-----BEGIN PUBLIC KEY-----\n$public_key\n-----END PUBLIC KEY-----\n";
    $public_key_id = openssl_pkey_get_public($public_key);
    if($public_key_id === false){
        return false;
    }
    //除去sign、sign_type两个参数外，凡是通知返回回来的参数皆是待验签的参数。
    $sign = $return_data['sign'];
    $sign_type = trim($return_data['sign_type'],'"');
    unset($return_data['sign'], $return_data['sign_type']);
 
    $query_string = signQueryString($return_data);
    $sign = base64_decode($sign);
    $rsa_verify = 0;
    if($sign_type == 'RSA'){
        $rsa_verify = openssl_verify($query_string, $sign, $public_key_id,OPENSSL_ALGO_SHA1);
    }else if($sign_type == 'RSA2'){
        $rsa_verify = openssl_verify($query_string, $sign, $public_key_id,OPENSSL_ALGO_SHA256);
    }
    openssl_free_key($public_key_id);
    if($rsa_verify == 0 || $rsa_verify == -1){
        //Returns 1 if the signature is correct, 0 if it is incorrect, and -1 on error.
        return false;
    }
    return $rsa_verify;
}