Just to add to the examples, if replacement is longer than length, only the length number of chars are removed from string and all of replacement is put in its place, and therefor strlen($string) is inreased.

$var = 'ABCDEFGH:/MNRPQR/';
/*  Should return ABCDEFGH:/testingRPQR/   */
echo substr_replace ($var, 'testing', 10, 2);