making my styleshett ccs file editable via a CP

I’m trying to make my styleshett ccs file editable via a Control Panel in the admin theme control panel theme option page, but it’s not working…. this is what i’ve done so far

1. renamed my css file to default.php in a “styles” foider
2. put this code in my header.php
<link rel="stylesheet" href="<?php bloginfo('template_directory'); ?>/styles/default.php" type="text/css" media="screen" />
3. I have a theme options page and i’ve added an extra input field so I can use that value…<?php echo $theme_options["css-alink"]; ?>… now if i place this echo code anywhere in my theme the value shows up fine, but the value won’t stick in my default.php style sheet when i place it in there eg.

this is my default.php file (shortened)

<?php
session_start();
header("Content-type: text/css");
?>

a { color:<?php echo $theme_options["css-alink"]; ?>;}

thanks for any help ??