One of the lovely things about php/asp etc is the ability to switch from html to script and back again (even within if statements). Your code would be much easier to manage if you took advantage of this.
Additionally html doesn't care if you use single or double quotes so your code could be rewritten as
if($ENABLE_SESSION){
echo "<frameset frameborder='0' border='0' framespacing='1' cols='*' rows='0,$mod_frame_size,*,160,0'>
<frame name='dummy src='dummy.html'>
<frame name='moderate' src='edit.$FILE_EXTENSION?.SID.' scrolling='no'>
<frame name='output' src='output.$FILE_EXTENSION' scrolling='auto'>
<frameset frameborder='0' border='0' framespacing='0' cols='*,210' rows='*'>";
Better still it could be rewritten as
if($ENABLE_SESSION){
?>
<frameset frameborder='0' border='0' framespacing='1' cols='*' rows='0,<?=$mod_frame_size?>,*,160,0'>
<frame name='dummy src='dummy.html'>
<frame name='moderate' src='edit.<?=$FILE_EXTENSION?>?.SID.' scrolling='no'>
<frame name='output' src='output.<?=$FILE_EXTENSION?>' scrolling='auto'>
<frameset frameborder='0' border='0' framespacing='0' cols='*,210' rows='*'>";
<?
and that probably looks a whole lot more familiar!