what is UX/UI design ?
please explian for me , i know only programming in php and javascript and css i know nothing about UX ,
is it for graphic designers ? or for programmers ?

UI/UX stands for User Interface / User Experience. Design is a big part of it, but it mostly means a great design that is usable, functional, and designed for an optimal experience for the end-user. It encompasses everything from color theory to the psychology behind why people click where they do and notice what they do, with the ultimate goal to be functional and have the highest potential conversion rates.

Nowadays the lines are very much blurred between graphic designers / web designers and the front-end developers (and even backend devs) since AJAX and other interactive elements play such an important role in the ultimate design / layout / functionality.

Yes but in the first stage a graphic designer who studied and well educated in the UX/UI well make easy life for the web designers and front end web developers right , then the UX/UI should be in the mind of the graphic designer who started the design. right?

User experience design (UX, UXD, UED or XD) is the process of enhancing user satisfaction with a product by improving the usability, accessibility, and pleasure provided in the interaction with the product.