Which one has the correct "lian", 炼功夫 or 练功夫? The phrase means &quot;to practice kung fu&quot;. Which one is correct?

炼功夫 (锻炼的炼)

or

练功夫 (练习的练)?

I've seen both versions floating about online but I've a feeling only one of them is correct and the other is just a common mistake, possibly even among native speakers.
According to Zhongwen Chinese Popup Dictionary, 炼 means "to refine, to smelt"; while 练 means "to practice, to train, to drill, to perfect (one's skills), exercise".

So obviously 练功夫 is more correct from a formal perspective, though both are possibly used informally.