Concatonate a string to PHP variable

I have the code below, and I want to add tn_ before each of the image names. I have tried to use concatenation but I keep getting PHP errors.

data-retina="<?php echo site_url('uploads/'.$gal->image); ?>"/>

Thanks,

Solution:

Do you mean this?

data-retina="<?php echo site_url('uploads/tn_'.$gal->image); ?>"/>