json解析出来为null，可能是utf8 bom头造成的

json_decode函数能够接收utf8编码的参数，但是当参数中包含BOM时，json_decode就会失效。
这个函数能将给定的字符串转换成UTF-8编码，移除其中的BOM。
下面是PHP代码：

function prepareJSON($input) {

    //This will convert ASCII/ISO-8859-1 to UTF-8.
    //Be careful with the third parameter (encoding detect list), because
    //if set wrong, some input encodings will get garbled (including UTF-8!)
    $imput = mb_convert_encoding($input, 'UTF-8', 'ASCII,UTF-8,ISO-8859-1');

    //Remove UTF-8 BOM if present, json_decode() does not like it.
    if(substr($input, 0, 3) == pack("CCC", 0xEF, 0xBB, 0xBF)) $input = substr($input, 3);

    return $input;
}

//Usage:
$myFile = file_get_contents('somefile.json');
$myDataArr = json_decode(prepareJSON($myFile), true);

文章来源：http://www.phpcode8.com/phpscripts/json-utf8-bom-solution.html