I have the follow code to change between different CSS theme files, I would like some reviews on it because I am sure it can be improved to be safer. <head>    <?php    $theme = $_GET['theme'];    $choice = preg_replace('/[^A-Za-z0-9\-]/', '', $theme);    if (empty($choice)){echo '<link relstylesheet type=text/css href=css/default.css />';}    elseif (file_exists(./css/ . $choice . .css)) {echo '<link relstylesheet type=text/css href=css/'.$theme.'.css />';}     else {echo '<link relstylesheet type=text/css href=css/default.css />';}    exit();    ?>    <!-- other elements in head -->    </head>The functions are:If GET string empty set CSS default fileIf GET non existing CSS file set CSS default fileIf GET existing CSS file them we use the echo GET.css file
PHP echo GET to change a CSS theme file
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