我正在尝试安装PHP5-sqlite3.在命令行中输入：
sudo apt-get install PHP5-sqlite3
结果我得到：
Reading package lists... Done Building dependency tree Reading state information... Done Package PHP5-sqlite3 is not available,but is referred to by another package. This may mean that the package is missing,has been obsoleted,or is only available from another source E: Package PHP5-sqlite3 has no installation candidate
这里发生了什么以及如何解决这个问题？
