php and css

<?php
// get first folder in URL
$f_folder = substr(substr($_SERVER["REQUEST_URI"],1), 0, strpos(substr($_SERVER["REQUEST_URI"],1), "/"));
//get full directory structure for current page
$full_path = rtrim(dirname($_SERVER['PHP_SELF']), '/\\');
#css for account
if ($f_folder='account') {
?>
    <link rel="stylesheet" href="/mainstyle/common.css" type="text/css" charset="utf-8" />
    <link rel="stylesheet" href="/mainstyle/account.css" type="text/css" charset="utf-8" />
<?php
} elseif ($f_folder='signin'||$f_folder='signup'||$full_path='/account/resetPassword'||$full_path='/account/logout') {
?>
    <link rel="stylesheet" href="/mainstyle/login-signup.css" type="text/css" charset="utf-8" />
<?php
} else {
?>
    <link rel="stylesheet" href="/mainstyle/common.css" type="text/css" charset="utf-8" />
<?php
}
?>