phacil-framework/system/ecompress/cssMin.php

<?php

/**
 * @param mixed $css 
 * @return string[]|string|null 
 */
function minimizeCSS($css){
    $css = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $css);
    $css = preg_replace('/\/\*((?!\*\/).)*\*\//', '', $css); // negative look ahead
    $css = preg_replace('/\s{2,}/', ' ', $css);
    $css = preg_replace('/\s*([:;{}])\s*/', '$1', $css);
    $css = preg_replace('/;}/', '}', $css);

    return $css;
}