dDw4NzEyMDk0MzQ7dDw7bDxpPDE%2BOz47bDx0PDtsPGk8Mj47aTw4Pjs%2BO2w8dDxwPHA8bDxUZXh0Oz47bDwyMDE0LTIwMTXnrKzkuIDlrabmnJ87Pj47Pjs7Pjt0PDtsPGk8MD47PjtsPHQ8O2w8aTwwPjtpPDE%2BO2k8Mj47PjtsPHQ8cDxwPGw8SG9yaXpvbnRhbEFsaWduO18hU0I7PjtsPFN5c3RlbS5XZWIuVUkuV2ViQ29udHJvbHMuSG9yaXpvbnRhbEFsaWduLCBTeXN0ZW0uV2ViLCBWZXJzaW9uPTEuMC41MDAwLjAsIEN1bHR1cmU9bmV1dHJhbCwgUHVibGljS2V5VG9rZW49YjAzZjVmN2YxMWQ1MGEzYTxSaWdodD47aTw2NTUzNj47Pj47Pjs7Pjt0PHA8cDxsPEhvcml6b250YWxBbGlnbjtfIVNCOz47bDw1MDxDZW50ZXI%2BO2k8NjU1MzY%2BOz4%2BOz47Oz47dDxwPHA8bDxIb3Jpem9udGFsQWxpZ247XyFTQjs%2BO2w8NTA8TGVmdD47aTw2NTUzNj47Pj47Pjs7Pjs%2BPjs%2BPjs%2BPjs%2BPjtsPDIxOzIyOzIzOzI0OzI1OzI2OzI3OzI4OzI5OzMwOzMxOzMyOzMzOzM0OzM1OzM2OzM3OzM4Oz4%2BzElOL0gk3AJyKWXnZbe4hXwMW3Y%3D

CSDN大牛回答：echo base64_decode(urldecode($s));

于是我先用UrlDecode解码（GB2312）  再用base64解码（GB2312）

得到结果

t<871209434;t<;l<i<1>;>;l<t<;l<i<2>;i<8>;>;l<t<p<p<l<Text;>;l<2014-2015第一学期;>>;>;;>;t<;l<i<0>;>;l<t<;l<i<0>;i<1>;i<2>;>;l<t<p<p<l<HorizontalAlign;_!SB;>;l<System.Web.UI.WebControls.HorizontalAlign, System.Web, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a<Right>;i<65536>;>>;>;;>;t<p<p<l<HorizontalAlign;_!SB;>;l<50<Center>;i<65536>;>>;>;;>;t<p<p<l<HorizontalAlign;_!SB;>;l<50<Left>;i<65536>;>>;>;;>;>>;>>;>>;>>;l<21;22;23;24;25;26;27;28;29;30;31;32;33;34;35;36;37;38;>>IN/H$r)ee|[v