How to check if string has at least one letter, number and special character in php

I am currently writing a small script that checks the contents of each string.

I was wondering what the REGEX would be to make sure that a string has a letter (upper or lower), a digit, and a special character?

Here is what I know so far (whcih isn't much):

if(preg_match('/^[a-zA-Z0-9]+$/i', $string)):

Help would be great!

Thank You!