keep getting a oauth_problem="parameter_absent:

cant figure how to fix this problem. can anyone help figure this problem please.
it says: Please provide valid credentials. OAuth oauth_problem="parameter_absent: oauth_signature", realm="yahooapis.com"

this is sample of the code i am using:
$apiurl = "http://social.yahooapis.com/v1/user/".$guid."/connections;start=0;count=max";
$params = "oauth_consumer_key=".$consumer_key;
$params .= "&oauth_nonce=".$nonce;
$params .= "&oauth_signature_method=HMAC-SHA1";
$params .= "&oauth_timestamp=".$timestamp;
$params .= "&oauth_token=".$request_token;
$params .= "&oauth_version=1.0";
$urlencoded_apiurl = urlencode($apiurl);
$urlencoded_params = urlencode($params);
$basestring = "GET&".$urlencoded_apiurl."&".$urlencoded_params;
$signature = base64_encode(hash_hmac("sha1", $basestring, $secret."&".$request_secret, true));
$url = "http://social.yahooapis.com/v1/user/".$guid."/connections;start=0;count=max";
$headers = array(
"Authorization: OAuth
realm=\"yahooapis.com\",
oauth_consumer_key=\"".$consumer_key."\",
oauth_nonce=\"".$nonce."\",
oauth_signature_method=\"HMAC-SHA1\",
oauth_timestamp=\"".$timestamp."\",
oauth_token=\"".$request_token."\",
oauth_version=\"1.0\",
oauth_signature=\"".$signature."\""
);