<?php

$prefix = '#someId';
$css = '#hello, .class{width:1px;height:1px;background-color:#AAA;}
div{font-size:1px}
input, a, span{padding:4px}';

$parts = explode('}', $css);
foreach ($parts as &$part) {
    if (empty($part)) {
        continue;
    }

    $subParts = explode(',', $part);
    foreach ($subParts as &$subPart) {
        $subPart = $prefix . ' ' . trim($subPart);
    }
    
    $part = implode(', ', $subParts);
}

$prefixedCss = implode("}\n", $parts);

echo $prefixedCss;

#someId #hello, #someId .class{width:1px;height:1px;background-color:#AAA;}
#someId div{font-size:1px}
#someId input, #someId a, #someId span{padding:4px}