Your HTML Code:
<? if (isset($_POST['submit'])) { $code = str_replace("<","<",$_POST['code']); $code = str_replace(">",">",$code); $code = str_replace('"','"',$code); echo $code; } ?>

Verify:

Optimized HTML Code:
<? $code = str_replace(" "," ",stripslashes($_POST['code'])); $code = str_replace("\r\n"," ",$code); $code = str_replace("<","<",$code); $code = str_replace(">",">",$code); $code = str_replace('"','"',$code); echo $code; ?> "; } } else { echo "Please input some HTML code before optimizing!
"; } } ?>