Mysql + GMAPs - 从DB中选择并计算距离

Question

我有MySQL表和一些库。我需要在PHP中进行SQL查询，它将按ID选择库，并计算距离。这是无效的查询。我在这个查询中做错了什么？

$query = "SELECT *,  
    ( 6371 * 
        acos(
            cos(radians(" . $center_lat . ")) *
            cos(radians(lat)) *
            cos(radians(lng) - radians(" . $center_lng . ")) +
            sin(radians(" . $center_lat . ") ) *
            sin(radians( lat ) )
        )
    ) AS distance
FROM `libraries`
WHERE id = ". $id ."
LIMIT 0 , 1";