substr_replace() replaces a copy of string delimited by the start and (optionally) length parameters with the string given in replacement. The result is returned.
If start is positive, the replacing will begin at the start'th offset into string.
If start is negative, the replacing will begin at the start'th character from the end of string.
If length is given and is positive, it represents the length of the portion of string which is to be replaced. If it is negative, it represents the number of characters from the end of string at which to stop replacing. If it is not given, then it will default to strlen( string ); i.e. end the replacing at the end of string.
Example 1. substr_replace() example