Can You Explain The Usage Of The Following Liabrary Functions:1.fopen 2.fflush 3.fclose 4.fgets?

These are file functions.

fopen takes the name of a file and how to open it (read-only, append, etc).  It returns a file handle.

fclose closes a file handle.

fflush pushes buffered data to the file.  This can be important if your OS delays writes to files for performance reasons.

fgets reads a line from a file handle.

For example, you might do this (in PHP):

$fh = fopen('myfile.txt','r');
$line = fgets($fh);
fclose($fh);

Error-checking is left as an exercise for the reader!

You don't need to use fflush unless you are writing to the file handle (with fputs for example).