Searches a string for occurrences of another string and returns the position in the first string where the first match is found.
Note: The first character in a string has index 0.
Parameter 1 - the string to search.
Parameter 2 - the string to search for.
Parameter 3 (Optional) - the position in the first string to start the search from. If omitted, the search starts from the beginning of the first string.
Returns: the parameter 3 value of the start of the first match found or -1 if the string could not be found.