bagadjan

AND (
  SELECT count(*)>0
  FROM s_variants pv
  WHERE pv.product_id=p.id AND pv.price>0
    AND (pv.stock IS NULL OR pv.stock>0) LIMIT 1
) = 1

AND p.id IN (
  SELECT product_id
  FROM s_variants
  WHERE price > 0 AND (stock IS NULL OR stock > 0)
)

preg_match_all('/(\w+)=(\w+)/', $str, $matches);
$data = array_combine($matches[1], $matches[2]);