Using PHP

Write a function wordFilter$($$str$)$ that takes a string representing a sentence as a parameter. It should replace all occurrences of certain blacklisted words with asterisks of the same length and return the substituted sentence. The words $$shoot$$darn$$ and $$fudge$($case insensitive$)$ are considered blacklisted. However, these words as part of a larger word should be left alone $($e$.$g$.$ overshoot is fine as it is$).$ Use one or more regular expressions with replacements in your solutions $($e$.$g$.$ preg$\_$replace$).$