Well you could put the returned string from highlight_file() in a textarea..... but this would put the file along with new HTML produced by PHP that colours the string. For example if your file had echo "hello";, the textarea could display <font color="black">echo</font>........ and so on. This can be done by using ob_start() etc. - check out thew manual at php,net for full details.
If however you want the text to actually be coloured withing the text area box then you need to use special components (DHTML) only avaiable to advanced browsers (IE5.5+). Its a pretty messy business.