An example that I used for tests.

define( 'API_ACCESS_KEY', 'fill_here_your_api_access_key' ); 
$registrationIds = array("fill_here_registered_id_of_your _device"); 

$msg = array ( 
    'parameter1' =>; 'Some data that you want to send',
    'parameter2' =>; 'Some additional data...',
);

$fields = array
(
    'registration_ids'  =>; $registrationIds,
    'data'              =>; $msg
);

$headers = array
(
    'Authorization: key=' . API_ACCESS_KEY,
    'Content-Type: application/json'
);

$ch = curl_init();
curl_setopt( $ch,CURLOPT_URL, 'https://android.googleapis.com/gcm/send' );
curl_setopt( $ch,CURLOPT_POST, true );
curl_setopt( $ch,CURLOPT_HTTPHEADER, $headers );
curl_setopt( $ch,CURLOPT_RETURNTRANSFER, true );
curl_setopt( $ch,CURLOPT_SSL_VERIFYPEER, false );
curl_setopt( $ch,CURLOPT_POSTFIELDS, json_encode( $fields ) );
$result = curl_exec($ch );
curl_close( $ch );

echo $result;