$json_data=file_get_contents('http://www.btc38.com/trade/gettradelist.php?coinname=xrp');$data=json_decode($json_data,true);var_dump($data);//为什么仍然输出字符串？？？？

ps:在jslint.com 测试$json_data为valid
回复内容： $json_data=file_get_contents('http://www.btc38.com/trade/gettradelist.php?coinname=xrp');$data=json_decode($json_data,true);var_dump($data);//为什么仍然输出字符串？？？？

ps:在jslint.com 测试$json_data为valid
bom！
我直接 echo 从网页获取到的内容，然后传递给 json_pp：
>>> php a.php | json_ppmalformed json string, neither array, object, number, string or atom, at character offset 0 (before \x{ef}\x{bb}\x{bf}{...) at /usr/bin/core_perl/json_pp line 44.

确实挺奇怪的，运行如上代码，使用json_last_error()最终提示的是语法错误。根据错误关键词搜索到so中的这个问题：json_decode returns json_error_syntax but online formatter says the json is ok 情况比较类似。按照答案之一说的怀疑了一下编码问题，不过看了下是utf-8，没有问题。
单纯的复制页面中的字符串的话是没有问题的（复制到页面和php中都没有问题）。这个不算是答案，算是帮助题主补充说明吧，我也还在努力的寻找答案中。
后记感谢 @依云 的答案，让我明白了原来是可恶的bom的问题，关于bom的问题 @依云 的维基百科链接已经很详细了，我就不多说了，这里就说一下怎么去除吧。其实bom就是在字符串的最开头增加了三个字符，我们把它去除掉就好了。在json_decode之前用substr去除就好了，例如：
$res = substr($res, 3);$arr = json_decode($res, true);
我这里没有问题啊：
{buyorder:[{price:0.222000,amount:63077.877631},{price:0.221000,amount:82921.415688},{price:0.220000,amount:53624.458032},{price:0.219000,amount:120232.956415},{price:0.218000,amount:58609.097966},{price:0.217000,amount:48330.724883},{price:0.216000,amount:15955.233203},{price:0.215000,amount:146085.153646},{price:0.214000,amount:17075.000000},{price:0.213000,amount:18729.000000}], sellorder:[{price:0.223000,amount:10724.828662},{price:0.224000,amount:133336.648858},{price:0.225000,amount:140519.661232},{price:0.226000,amount:10095.282427},{price:0.227000,amount:26586.879929},{price:0.228000,amount:27247.504336},{price:0.229000,amount:35233.324750},{price:0.230000,amount:84477.240158},{price:0.231000,amount:17260.931218},{price:0.232000,amount:72333.275935}], trade:[{price:0.223000,volume:52.322237,time:2013-12-12 18:41:21,type:1},{price:0.222000,volume:1.000000,time:2013-12-12 18:41:02,type:2},{price:0.223000,volume:849.956784,time:2013-12-12 18:37:45,type:1},{price:0.223000,volume:8181.433350,time:2013-12-12 18:37:45,type:1},{price:0.223000,volume:1696.845442,time:2013-12-12 18:34:42,type:1},{price:0.223000,volume:63.826185,time:2013-12-12 18:34:42,type:1},{price:0.222000,volume:322.039509,time:2013-12-12 18:33:57,type:2},{price:0.222000,volume:676.960491,time:2013-12-12 18:33:57,type:2},{price:0.222000,volume:3628.741136,time:2013-12-12 18:33:40,type:2},{price:0.223000,volume:0.672054,time:2013-12-12 18:33:31,type:1},{price:0.222000,volume:1.000000,time:2013-12-12 18:33:11,type:2},{price:0.223000,volume:442.969488,time:2013-12-12 18:32:41,type:1},{price:0.222000,volume:2.455251,time:2013-12-12 18:31:49,type:2},{price:0.223000,volume:20.715520,time:2013-12-12 18:28:48,type:1},{price:0.223000,volume:465.314511,time:2013-12-12 18:26:54,type:1},{price:0.223000,volume:6.502242,time:2013-12-12 18:26:50,type:1},{price:0.224000,volume:35.229334,time:2013-12-12 18:22:31,type:1},{price:0.224000,volume:100.000000,time:2013-12-12 18:22:13,type:1},{price:0.223000,volume:1609.883424,time:2013-12-12 18:22:07,type:2},{price:0.223000,volume:99.900000,time:2013-12-12 18:21:35,type:2}]}
保存到json_file
然后
var_dump(json_decode(file_get_contents('json_file'), true));
结果是
array(3) { [buyorder]=> array(10) { [0]=> array(2) { [price]=> string(8) 0.222000 [amount]=> string(12) 63077.877631 } [1]=> array(2) { [price]=> string(8) 0.221000 [amount]=> string(12) 82921.415688 }以下省略...
可能是服务器不太稳定，传输的数据错误了。
求截图...