about changing filename

Hi,

I have client who uploaded images with space bar. I can run a PHP file to change the filename at database, so the space bar will be changed to -. Its very simple by using strtr.

But the question is, how to change the real image file?
for example, the image name stored at folder images/ is abd ef.jpg. It should be changed to abd_ef.jpg, so there's no space bar at the real image name. Not only change at database mysql.

Thanks.