验证谷歌付费签名？

Question

我在php中使用openssl_verify验证来自我的应用程序的签名

<?php
// $data and $signature are assumed to contain the data and the signature

// fetch public key from certificate and ready it
$pubkeyid = openssl_pkey_get_public("file://src/openssl-0.9.6/demos/sign/cert.pem");

// state whether signature is okay or not
$ok = openssl_verify($data, $signature, $pubkeyid);
if ($ok == 1) {
    echo "good";
} elseif ($ok == 0) {
    echo "bad";
} else {
    echo "ugly, error checking signature";
}
// free the key from memory
openssl_free_key($pubkeyid);
?>

我的php内置函数需要$ pubkeyid作为文件pem但谷歌发送给我一个公钥作为字符串。他们说Note:To find the public key portion of this key pair, open your application's details in the Developer Console, then click on Services & APIs, and look at the field titled Your License Key for This Application.

我的问题是如何从publickey String创建文件cert.pem（google发给我的公钥） 非常感谢你！