Regular expression help

I just plain suck at regex, so I was hoping to get some hints from you regex experts out there. I'm sure it's been done a thousand times, but I can't seem to find what I'm looking for with a simple google search: $phone could be "1234567890" OR $phone could be "123-456-7890" OR $phone could be "(123) 456 7890 OR $phone could have other unexpected characters in it, even control characters. So what I'm desiring to do is run some type of regex on $phone and only return the integers and assign the cleaned string back to $phone. I thought I knew how to do this, but it's not working. Thanks for your guidance.