banana apple mango
( слова разделены пробелами)*banana
*apple
*mango
$string = 'banana apple mango';
$arr = explode(' ', $string);
$result = array_map(function($v){
return '*' . $v;
}, $arr);
var_dump($result);
/*
array(3) {
[0]=>
string(7) "*banana"
[1]=>
string(6) "*apple"
[2]=>
string(6) "*mango"
}
*/