Using Amazon SQS Rest API in PHP: AccessDenied error - php

Question

I have an app running on PHP 5.3 and it needs to use Amazon SQS. I must use AWS REST API directly, and not AWS PHP SDK, since the recent SDKs do not run on PHP 5.3, and the older SDKs do not support the current API parameters of SQS.
I am trying a simple SendMessage API call with a minimal script, which is attached below. It always shows the following error, and no SQS message is sent, of course.
Code: 'AccessDenied' and Message: 'Access to the resource https://sqs.us-east-1.amazonaws.com/999999999999/test1.fifo is denied'.
1) I have primarily followed these docs, and a sample Python script available there, for writing this code.
a) https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-working-with-apis.html
b) https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html
2) I have checked these critical variables in my code several times and they are correct: endpoint, host, region, key, secrete, uri etc.
3) I have used those same variables in AWS PHP SDK for SQS on another computer running a recent version of PHP, and that has worked successfully.
UPDATE >> 4) The access id and secrete key used in the code belong to an IAM user who has full AWS Administrative privilege.
/////// COMPLETE CODE //////////////
$method = 'POST';
$service = 'sqs';
$endpoint = 'https://sqs.us-east-1.amazonaws.com/999999999999/test1.fifo';
$host = 'sqs.us-east-1.amazonaws.com';
$region = 'us-east-1';
$key = 'XXXXXXXXXXXXXXXXX';
$secret = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
$content_type = 'application/x-www-form-urlencoded';
$amz_date = gmdate('Ymd').'T'.gmdate('His').'Z';
$date_stamp = gmdate('Ymd');
//post fields for SendMessage()
$message_group_id = 1;
$message_deduplication_id = $amz_date;
$message = 'An awesome message sent at: ' . $amz_date;
$post_data = 'Action=SendMessage&Version=2012-11-05&MessageGroupId='. urlencode($message_group_id) .'&MessageDeduplicationId=' . urlencode($message_deduplication_id) . '&MessageBody=' . urlencode($message);
//CANONICAL REQUEST
$canonical_uri = '/999999999999/test1.fifo';
$canonical_querystring = '';
$canonical_headers = 'content-type:' . $content_type . '\n' . 'host:' . $host . '\n' . 'x-amz-date:' . $amz_date . '\n';
$signed_headers = 'content-type;host;x-amz-date';
$payload_hash = hash('sha256', utf8_encode($post_data));
$canonical_request = $method . '\n' . $canonical_uri . '\n' . $canonical_querystring . '\n' . $canonical_headers . '\n' . $signed_headers . '\n' . $payload_hash;
//STRING TO SIGN
$algorithm_name = 'AWS4-HMAC-SHA256';
$algorithm = 'sha256';
$credential_scope = $date_stamp . '/' . $region . '/' . $service . '/' . 'aws4_request';
$string_to_sign = $algorithm_name . '\n' . $amz_date . '\n' . $credential_scope . '\n' . hash('sha256', utf8_encode($canonical_request));
//SIGNATURE
$dateKey = hash_hmac($algorithm, $date_stamp, utf8_encode('AWS4' . $secret), true);
$dateRegionKey = hash_hmac($algorithm, $region, $dateKey, true);
$dateRegionServiceKey = hash_hmac($algorithm, $service, $dateRegionKey, true);
$signingKey = hash_hmac($algorithm, 'aws4_request', $dateRegionServiceKey, true);
$signature = hash_hmac('sha256', $string_to_sign, $signingKey, false);
//REQUEST
$authorization_header = $algorithm_name . ' ' . 'Credential=' . $key . '/' . $credential_scope . ', ' . 'SignedHeaders=' . $signed_headers . ', ' . 'Signature=' . $signature;
$http_header = array(
'Content-Type' => $content_type,
'X-Amz-Date' => $amz_date,
'Authorization' => $authorization_header
);
//SEND
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $endpoint);
curl_setopt($curl, CURLOPT_HTTPHEADER, $http_header);
curl_setopt($curl, CURLOPT_HEADER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $post_data);
if ($response = curl_exec($curl)) {
$response_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
echo '<br><br><br>Response code: ' . $response_code;
$xml = simplexml_load_string($response);
print_r($xml);
}
else {
$error_code = curl_errno($curl);
$error_message = curl_error($curl);
echo '<br><br><br>Error: ' . $error_code . ' => ' . $error_message;
}
#curl_close($curl);
Any observation/suggestion as to what is causing the error will be appreciated.