Thanks for reply.
I've used JAVA, but am learning to use PHP and so have installed LAMP on the family's Mint 7 laptop. (Good God, it's so easy to do!) I also have msttcorefonts installed.
I do lots of DTP and, you're right, I don't use the vast majority of the fonts I have. Nevertheless, I do like scouting through them occasionally, if I can't find one that's 'just right' for what I want in the one's I regularly use. I thought it would be possible to 'dress up' any LAMP apps I wrote for myself once I got proficient enough. Any I wrote that I might want to show off to others would need to be written with the core fonts in mind.
I've tried a couple more times just cp-ing the fonts in and updating the cache, and still can't do it without it messing up the other's browsers. Even after logout-login, or restart. My browser is always unaffected.
Dropping the fonts into home/.fonts stops them working in any PHP pages I produce.
However...
I decided to try downloading some different fonts, from
http://www.1001freefonts.com, put them into /usr/share/fonts, and recached. Bingo everything still works and I can use the fonts in my PHP pages.
I did an ls of the directory I have archived all the fonts in, though, and noticed the three new fonts were displayed differently. (See picture)

- Screenshot.png (14.27 KiB) Viewed 25 times
I still don't know enough about file permissions, being newer than some to Linux, but as far as I know the font used in the terminal is dependent on various file properties.
Thinking that it might be the permission present on the other fonts, I tried copying just one across...
It worked.
I now have a funny feeling that, until I more fully understand file permissions, I'm only going to be able to do this one font at a time, checking as I go.
Am I on the right track?