It would be extremely helpful to be able to do more formatting of numbers than is currently available.  
Specifically, it would be good if number_format would prepend a dollar sign or allow negative numbers to be shown in brackets rather than with a negative sign.  Both of these are necessary for accounting applications.

My current solution for negative currency looks like this: 
  if ( $value < 0 )
  {
    $value = "(\$".number_format(abs($value),2).")";
  }
which works but is less than ideal.  For one thing, it makes later use of the number difficult.