生成签名的时候，openssl_sign返回false，这是什么原因导致的呢

$url_parts = parse_url($url);
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$canonical_url = ($url_parts['path'] . (!empty($url_parts['query']) ? "?${url_parts['query']}" : ""));
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$message = $http_method."\n".
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$canonical_url."\n".
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$timestamp."\n".
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$nonce."\n".
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$body."\n";
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openssl_sign($message, $raw_sign, $mch_private_key, 'sha256WithRSAEncryption');
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$sign = base64_encode($raw_sign);
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$schema = 'WECHATPAY2-SHA256-RSA2048';
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$token = sprintf('mchid="%s",nonce_str="%s",timestamp="%d",serial_no="%s",signature="%s"',
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$merchant_id, $nonce, $timestamp, $serial_no, $sign);