MichaelNi on "wp_enqueue_style() with php file and css styles (php functions do not work)"

I'm just developing my first own plugin; I enqueued a .css.php file and used <?php header("Content-type: text/css"); ?> to specify that the content is CSS via wp_enqueue_style(). However, the wordpress PHP functions do not seem to work. I have two files, both are quit with the following errors:

<b>Fatal error</b>: Call to undefined function plugins_url() in <b>/(...)/wp-content/plugins/simple-self-stylable-popup/ssspu_admin_css.php</b> on line <b>27</b><br />

<b>Fatal error</b>: Call to undefined function get_option() in <b>/(...)/wp-content/plugins/simple-self-stylable-popup/ssspu_css.php</b> on line <b>5</b><br />

I tried googeling, but can't find any usable answers.