This function returns the string that emerges if in the string all occurrences of search_string are replaced by replace_ string.
- If replace_string is omitted or if it is NULL, all occurrences of search_string are deleted from the result.
- If search_string is NULL, string is returned.
- If the input parameters are not strings, they will be automatically converted to strings.
- The return type is always a string, even if all of the parameters possess another type.