To check if a a steam profile is ingame and if so which game use the following function as a guide:

function ingame($steamid)
{
    $data = json_decode(file_get_contents("https://api.steampowered.com/ISteamUser/GetPlayerSummaries/v0002/?key=YOURAPIKEY&steamids=" . $steamid . ""));
    $main = $data->response->players[0];
    if (is_null($main->gameid)) {
         //Not in a game
        return "Not ingame";
    } else {
        $gamename = $main->gameextrainfo;
        $gameid = $main->gameid;
        return "".$gamename." ".$gameid."";
    }
}

echo ingame('765876594806');

Dont forget to add your setam api key where it says YOURAPIKEY