how to generate a random 6 digit alphanumeric string with 3 alphabet and 3 numerals in it using php

I have a code in PHP that is able to generate a 6 digit alpha numeric string but it does not ensure that the letter generated are 3 and the numbers generated are 3. It generates a string such as "ps7ycn" It does not have 3 numbers in between the alphabet. The numbers should be in between the letters. exammple : a3g5h9