PHP function to replace a (i)th-position character

Question

Is there a function in PHP that takes in a string, a number (i), and a character (x), then replaces the character at position (i) with

Answer 1

implode(':', str_split('1300', 2));

returns:

13:00

Also very nice for some credit card numbers like Visa:

implode(' ', str_split('4900000000000000', 4));

returns:

4900 0000 0000 0000

str_split — Convert a string to an array