loadVars send always return undefined

programming flash
1

Please help!
I’ve searched all forums for solution and didn’t find any appropriate.
Flash file:

s.onSoundComplete = function(){
      loadVars_out.songId = sa[cps].id;
      loadVars_out.send("incPlayedNumber.php", "_self", "POST");
      playNext();}

trace(loadVars_out.songId); returns correct numbers, IDs, typeof(loadVars_out.songId) is string.
php file:

$songId = $_POST['songId'];
echo "$songId";

PHP is called correctly, but $sondId always undefined. Flash 8, player version is 10.
Any suggestions?