PHP inserts a string into a random position of a string

/**
* Randomly inserts a string into a string
* @param unknown_type $oldstr old string
* @param unknown_type $instr the inserted string
* @param unknown_type $encoding string encoding
*/
function Rand_in_str ($oldstr, $instr, $encoding) {
$len = Mb_strlen ($oldstr, $encoding);
$insert _point = Mt_rand (1, $len-1);
$pre _str = mb_substr ($oldstr, 0, $insert _point, $encoding);
$after _str = Mb_substr ($oldstr, $insert _point, $len-$insert _point, $encoding);
$newstr = $pre _str. $instr. $after _str;
return $newstr;
}