Display css with file_get_contents to avoid rendering blocking

Question

I’m optimizing a site (which has already improved a lot) and in Google’s Pagespeed Tools, says I have to "Eliminate Javascript and CSS rendering blocking content above the edge".

It worked out that way:

<style style="text/css">
  <?php
  /* Sweet alert */
  
  echo file_get_contents('js/sweetalert-master/dist/sweetalert.css');
  ?>
</style>

With file_get_contents in PHP. But the sources, like this:

<link rel="stylesheet" href="fonts/open_sans/stylesheet.css">

file_get_contents does not work.

I can put the link of the fonts in the footer, but the site loads without the fonts and does not get a cool aesthetic.