str_replace

I am trying to figure out a syntax that will replace each instance of % with a different letter chosen randomly from the string $puzzle_filler. $puzzle_filler is populated with the letters of the alphabet, roughly in the same ratio as they are used. This syntax replaces each instance of % with the same letter: $puzzle[$i] = str_replace ( "%" , ( substr ( $puzzle_filler , rand(1,98) , 1 ) ) , $puzzle[$i] ); Turning this: %ECARBME%TIPLUP%%%%%%%E%% Into: uECARBMEuTIPLUPuuuuuuuEuu Is there a way to tweak my str_replace so it will only do 1 % at a time, so a different replacement letter is selected? This is the syntax specific to choosing a replacement letter at random: substr ( $puzzle_filler , rand(1,98) , 1 ); Thanks for your help. Ron The Verse of the Day “Encouragement from God’s Word” http://www.TheVerseOfTheDay.info