how can i creat auto sub page as like as facebook

Hello walishoeb,

what kind of sub page do you mean? Something like the little Chat window?

This is typically done by using a Javascript function that "unhides" a DIV element that was already present on the page and filling it using AJAX calls. Of course it is also possible to create a new such element of the page using Javascript.

Its not exactly a PHP topic though.

To make things more convenient for the developer, there are some popular Javascript libraries / frameworks available that simplify the coding. Examples for that are mootools or jquery.