String Manipulation

["Chris Payne" <chris@planetoxygene.com>]

Hi there everyone,

How can I easily change specific characters of a string?  I tried the following:

 $passroutine = str_replace("a", "z", $passroutine);

But it doesn't seem to work?  Basically it needs to replace letters throughout the whole string - whether a seperate word such as A or within a word such as Abra.

Any help would be greatly appreciated :-)

Thanks

Chris