Check if string1 is part of the substring for string2

Is that any function to check if a certain string is part of the another string ?? For example, i want to check if a sting1 is the substring of string 2 or not .. Let's say string1 = "mum"; and string2 = "minimum"; If this was the case then string1 is part of the substring for string2, so the result will return true.. Is that anyway to do this ???