Theme check warning

Hallo,

my theme get on Theme Check the following Warning.

WARNING: file_get_contents was found in the file js/theme.js.php File operations should use the WP_Filesystem methods instead of direct PHP filesystem calls.

Line 23: echo file_get_contents($jss);

I use this script for compressing my css and javascript…

<?php
/**
 * Pressonry dynamic All in One Styles
 * www.phpdevtips.com
 *
 * @package pressonry
 */
header("Content-Type: text/css;charset: UTF-8");
function compress($buffer) {
	$buffer = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $buffer);
	$buffer = str_replace(array("\r\n", "\r", "\n", "\t", '  ', '    ', '    '), '', $buffer);
	return $buffer;
}
$get_styles = array(
	'bootstrap.min.css',
	'theme-plugins.css',
	'theme-custom.css'
);
foreach($get_styles as $sheet) {
	$sheets = trim($sheet);
	if(file_exists($sheets)) {
		ob_start("compress");
		echo file_get_contents($sheets);
		ob_end_flush();
	}
}
?>

Can someone tell me how I can write this code wordpress friendly.

Thanks in advance
Marc