I'm not sure if you have worked on this Regex pattern to do something unique. However, I will provide you with a general one that should fit your question without using your current pattern.
$my_string = '88888805';
echo preg_replace("/([0-9]+)[0-9]{2}$/","$1**",$,my_string);
Explanation:
The ([0-9]+) will match all digits, this could be replaced with \d+, it's between brackets to be captured as we are going to use it in the results.
[0-9]{2} is going to match the last 2 digits, again, it can be replaced with \d{2}, it's outside the brackets because we don't want to include them in the result. the $ after that is to indicate the end of the test, it's optional anyways.
Results:
Input: 88888805
Output: 888888**