// Array of css files
$css = array(
    'main.css',
    'menu.css',
    'content.css'
);

// Prevent a notice
$css_content = '';

// Loop the css Array
foreach ($css as $css_file) {
    if(file_exists($css_file)){
    // Load the content of the css file 
    $css_content .= file_get_contents($css_file);
    $css_content = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $css_content);
            /* remove tabs, spaces, newlines, etc. */
    $css_content = str_replace(array("\r\n", "\r", "\n", "\t", '  ', '    ', '    '), '', $css_content);
}
}

// print the css content
echo $css_content;