Способ сделать это:
$old = 'Ha.pp.yB.ir.th.da.y';
$arr = str_split($old); #break string into an array
#iterate over the array, but only do it over the characters which are a
#multiple of three (remember that arrays start with 0)
for ($i = 2; $i < count($arr); $i+=2) {
#remove current array item
array_splice($arr, $i, 1);
}
$new = implode($arr); #join it back
Или с регулярным выражением:
$old = 'Ha.pp.yB.ir.th.da.y';
$new = preg_replace('/(..)\./', '$1', $old);
#selects any two characters followed by a dot character
#alternatively, if you know that the two characters are letters,
#change the regular expression to:
/(\w{2})\./