First, you need to explode your array. That will create an indexed array, which starts the index from 0 up to 2. Next, you will need to implode it in some manner. Other answers are more explicit and use foreach, so this one provides an alternative, where you convert your array into the desired format via array_map. The function passed to array_map is a so-called callback, a function that is executed by array_map for each element, in this case, which is aware of the key and the value of each element. Now, we implode our input and, to make sure that we have some keys, we pass the result of array_keys.
$input = explode(",", "Hello,Howdy,Hola");
$output = implode('<br>', array_map(
function ($v, $k) {
return ($k + 1).'. '.$v;
},
$input,
array_keys($input)
));
implodeto transform your array into a string again, with whatever needs to be appended between the individual elements.