Q:

Allow highlighting certain bits of a code fragment or quote

It would be nice to be able to put emphasis on specific bits of a fragment or of a quote (bold can help for a quote but I find that it makes things less readable sometimes).
What do you think?
Update: On http://www.pastebin.com/, you can highlight particular lines by prefixing each line with @@.
See also:

Bold code in a question

A:

I like the idea.
For highlighting a whole line, one could extend the markdown syntax and define that an asterisk before the four spaces required to highlight a code block highlights that line:
    for ($i = 0; $i <= 1; $i++)
     {  
*      print "$i lines!";  // this line gets highlighted
     }

this would work without having to work out a character or character sequence that turns on highlighting within a block.

A:

What about just breaking up the code block?

You need to use the print function:
function foobar()
{
  for ($i = 0; $i <= 1; $i++)
  { 

    print "$i lines!";

  }
}

