why Getting Acces Denied while uploading file using aws-sdk-php?

plz i'm somehow lost here , i'm new in aws so i've installed aws-sdk-php in myproject , and i'm trying to test the upload ; here is the peace of code of the script :

require '.vendor/autoload.php';
use Aws\S3\S3Client;
use Aws\S3\Exception\S3Exception;

// Instantiate an S3 client
$s3 = S3Client::factory(array(
    'credentials' => array(
        'key'    => 'key ... ',
        'secret' => 'secret key ... ',
    )
)); 
$file_name = $_FILES['file']['tmp_name'];
try {
    $s3->putObject(array(
        'Bucket' => 'bucket_name',
        'Key'    => $_FILES['file']['name'],
        'Body'   => fopen($file_name, 'r'),
        'ACL'    => 'public-read',
    ));
    echo " upload succed !!";
} catch (S3Exception $e) {
    echo "There was an error uploading the file.\n";
    echo $e->getMessage() . "\n";
}

this code is working fine from a local test , but when trying to do test the same script in a project in a folder in s3 ; i get this error :

There was an error uploading the file. Access Denied

any idea plz ? thanks.