composer require aws/aws-sdk-php

$namespaceName = 'frpegp***';
$bucketName = 'test******05';
$region = 'eu-frankfurt-1';
$endpoint = "https://$namespaceName.compat.objectstorage.$region.oraclecloud.com";
$s3 = new Aws\S3\S3Client([
    'version' => 'latest',
    'region'  => $region,
    'endpoint' => $endpoint,
    'signature_version' => 'v4',
    'use_path_style_endpoint' => true,
    'credentials' => [
        'key'    => 'AKI***YYJ', // remove if you have env var AWS_ACCESS_KEY_ID
        'secret' => 'ndK***cIf', , // remove if you have env var AWS_SECRET_ACCESS_KEY
    ],
]);
$cmd = $s3->getCommand('GetObject', [
    'Bucket' => $bucketName,
    'Key' => 'fff.txt'
]);
$request = $s3->createPresignedRequest($cmd, '+20 minutes');

<?xml version="1.0" encoding="UTF-8"?>
<Error>
    <Message>The required information to complete authentication was not provided.</Message>
    <Code>SignatureDoesNotMatch</Code>
</Error>

composer require hitrov/oci-api-php-request-sign

require 'vendor/autoload.php';
use Hitrov\OCI\Signer;

OCI_TENANCY_ID=ocid1.tenancy.oc1..aaaaaaaaba3pv6wkcr4jqae5f15p2b2m2yt2j6rx32uzr4h25vqstifsfdsq
OCI_USER_ID=ocid1.user.oc1..aaaaaaaat5nvwcna5j6aqzjcaty5eqbb6qt2jvpkanghtgdaqedqw3rynjq
OCI_KEY_FINGERPRINT=20:3b:97:13:55:1c:5b:0d:d3:37:d8:50:4e:c5:3a:34
OCI_PRIVATE_KEY_FILENAME=/path/to/privatekey.pem

$signer = new Signer;

public function getHeaders(
    string $url, string $method = 'GET', ?string $body = null, ?string $contentType = 'application/json', string $dateString = null
): array

$curl = curl_init();
$url = 'https://objectstorage.eu-frankfurt-1.oraclecloud.com/n/{namespaceName}/b/{bucketName}/p/';
$method = 'POST';
$body = '{"accessType": "ObjectRead", "name": "read-access-to-image.png", "objectName": "path/to/image.png", "timeExpires": "2021-03-01T00:00:00-00:00"}';
$headers = $signer->getHeaders($url, $method, $body, 'application/json');
var_dump($headers);
$curlOptions = [
  CURLOPT_URL => $url,
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 5,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => $method,
  CURLOPT_HTTPHEADER => $headers,
];
if ($body) {
  // not needed for GET or HEAD requests
  $curlOptions[CURLOPT_POSTFIELDS] = $body;
}
curl_setopt_array($curl, $curlOptions);
$response = curl_exec($curl);
echo $response;
curl_close($curl);

array(6) {
  [0]=>
  string(35) "date: Mon, 08 Feb 2021 20:49:22 GMT"
  [1]=>
  string(50) "host: objectstorage.eu-frankfurt-1.oraclecloud.com"
  [2]=>
  string(18) "content-length: 76"
  [3]=>
  string(30) "content-type: application/json"
  [4]=>
  string(62) "x-content-sha256: X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE="
  [5]=>
  string(538) "Authorization: Signature version="1",keyId="ocid1.tenancy.oc1..aaaaaaaaba3pv6wkcr4jqae5f15p2b2m2yt2j6rx32uzr4h25vqstifsfdsq/ocid1.user.oc1..aaaaaaaat5nvwcna5j6aqzjcaty5eqbb6qt2jvpkanghtgdaqedqw3rynjq/20:3b:97:13:55:1c:5b:0d:d3:37:d8:50:4e:c5:3a:34",algorithm="rsa-sha256",headers="date (request-target) host content-length content-type x-content-sha256",signature="LXWXDA8VmXXc1NRbMmXtW61IS97DfIOMAnlj+Gm+oBPNc2svXYdhcXNJ+oFPoi9qJHLnoUiHqotTzuVPXSG5iyXzFntvkAn3lFIAja52iwwwcJflEIXj/b39eG2dCsOTmmUJguut0FsLhCRSX0eylTSLgxTFGoQi7K/m18nafso=""
}

{
  "accessUri": "/p/AlIlOEsMok7oE7YkN30KJUDjDKQjk493BKbuM-ANUNGdBBAHzHT_5lFlzYC9CQiA/n/{namespaceName}/b/{bucketName}/o/path/to/image.png",
  "id": "oHJQWGxpD+2PhDqtoewvLCf8/lYNlaIpbZHYx+mBryAad/q0LnFy37Me/quKhxEi:path/to/image.png",
  "name": "read-access-to-image.png",
  "accessType": "ObjectRead",
  "objectName": "path/to/image.png",
  "timeCreated": "2021-02-09T11:52:45.053Z",
  "timeExpires": "2021-03-01T00:00:00Z"
}

$signingHeadersNames = $signer->getSigningHeadersNames('POST');

$bodyHashBase64 = $signer->getBodyHashBase64($body);

date: Mon, 08 Feb 2021 20:51:33 GMT
(request-target): post /n/{namespaceName}/b/{bucketName}/p/
host: objectstorage.eu-frankfurt-1.oraclecloud.com
content-length: 76
content-type: application/json
x-content-sha256: X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

$signingString = $signer->getSigningString($url, $method, $body, 'application/json');

$signature = $signer->calculateSignature($signingString, $privateKeyString);

$keyId = $signer->getKeyId();

$signingHeadersNamesString = implode(' ', $signingHeadersNames);
$authorizationHeader = $signer->getAuthorizationHeader($keyId, $signingHeadersNamesString, $signature);

Authorization: Signature version="1",keyId="ocid1.tenancy.oc1..aaaaaaaaba3pv6wkcr4jqae5f15p2b2m2yt2j6rx32uzr4h25vqstifsfdsq/ocid1.user.oc1..aaaaaaaat5nvwcna5j6aqzjcaty5eqbb6qt2jvpkanghtgdaqedqw3rynjq/20:3b:97:13:55:1c:5b:0d:d3:37:d8:50:4e:c5:3a:34",algorithm="rsa-sha256",headers="date (request-target) host content-length content-type x-content-sha256",signature="LXWXDA8VmXXc1NRbMmXtW61IS97DfIOMAnlj+Gm+oBPNc2svXYdhcXNJ+oFPoi9qJHLnoUiHqotTzuVPXSG5iyXzFntvkAn3lFIAja52iwwwcJflEIXj/b39eG2dCsOTmmUJguut0FsLhCRSX0eylTSLgxTFGoQi7K/m18nafso="