echo数组单元的值不全

数组的为

Array(    [title] => 初一上册数学测试题    [id] => 6    [subject] => 14    [parts] => [["单选","100",1]]    [topic] => [["1","100"]]    [json] => [["6","
小红做一道数学题“ ';color:rgb(0,0,0);font-weight:bold;font-size:14px;font-family:    [#39;'>两个多项式 'font-family:Times New Roman'>A、B，B为，试求 'font-family:Times New Roman'>A B的值”。小红误将 'font-family:Times New Roman'>A B看成A－ ';color:rgb(0,0,0);font-size:14px;font-family:    [#39;'>B 'font-family:宋体'>，结果答案（计算正确）为．
 'text-indent:21px;line-height:200%'>（ 'font-family:Times New Roman'>2）求出当x ';color:rgb(0,0,0);font-size:14px;font-family:    [#39;'>] => 3时A B的值.

",1,"100"]]    [total] => 100)

我在打印其中的json键时，却没打印全，显示

[["6","