I know how to read a file on the server and attach it to an email in PHP, but I wanted to know if I could attach a file that is created by my script but not created on the server (kinda like a temp file).
So create file in memory and attach it to email.
Bonus: might need to create multiple files as well, would this be too much for the server to handle? I'm not talking GB's but like 5 files with 1000 lines each?
Yes you can do that, as long as whatever email library you're using supports it. If you're not using one, you should be!
No, 5 files won't be too much for your server unless you bought it in 1993.
Hopefully your lib won't need a file reference - you can do something like:
$myEmail->attachData('file.name', 'mime/type', $data);
If it does need a file path then you could use a php://memory file:
$f = fopen('php://memory/myfile', 'w');
fwrite($f, '...');
fclose($f);
$myEmail->attach('php://memory/myFile');