I am not very experienced in php. I have following problem:

I am uploading a picture and during upload I create a new resized picture. I would like to rename this picture like that:
s_filename.jpg > x_filename.jpg

this script is only setting the x_ as prefix. so I get x_s_filename.jpg, but I need only x_filename.jpg

My script:
PHP Code:
how can I manage this?

I looking forward for any help!