string replace(string s, string from, string to);

When given strings as second and third argument, a copy of
s with every occurance of 'from' return 'to' is returned.

string replace(string s, string *from, string *to);

When given arrays of strings as second and third argument,
every occurance of from[0] in s is replaced by to[0],
from[1] is replaced by to[1] and so on...

array replace(array a, mixed from, mixed to);
mapping replace(mapping a, mixed from, mixed to);

When the first argument is an array or mapping, the values in
a are searched for values equal to from, which are replaced by
to destructively.