数据库获取josn字符串如何解析

从数据库中获取的json字符串赋给一个变量，再用json_decode()解析变量得到null,如果直接用字符串就可以解析成json对象，请问是为何？这是输出的字符串
[
    {
        "school": "u5317u4eacu5927u5b66",
        "start": "09-2004",
        "end": "06-2008",
        "qualifications": "u5b66u58eb",
        "qualifications_date": "01-06-2008"
    },
    {
        "school": "",
        "start": "",
        "end": "",
        "qualifications": "",
        "qualifications_date": ""
    }
]