What should I do if a package has not installation candidates?

I am trying to install php5-sqlite3. In the command line I type:

sudo apt-get install php5-sqlite3

As I result I get:

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

What is happening here and how I can solve this problem?

From serverfault Roman

• I think you want php5-sqlite.

  Roman : Thank you. It worked. I was able to install php5-sqlite and after that I was able to use sqlite from PHP without any additional changes.
  From alex

• You could have found the package you were searching for by using:

  $ sudo apt-cache search php sqlite
  

  More information on the search command from the apt-cache man page:

  search regex [ regex ... ]

  search performs a full text search on all available package lists for the regex pattern given. It searches the package names and the descriptions for an occurrence of the regular expression and prints out the package name and the short description. If --full is given then output identical to show is produced for each matched package, and if --names-only is given then the long description is not searched, only the package name is.

  Separate arguments can be used to specify multiple search patterns that are anded together.

  From mrucci