I have index.php file, in my dating site, but, I want that the file mydating.php should be an index file

You can do this by using the .htaccess file

Alternate index files are entered in a list. The server will work from left to right, checking to see if each file exists, if none of them exisit it will display a directory listing (unless, of course, you have turned this off).

DirectoryIndex mydating.php index.php index.html index.htm