src/Primitive/String/String.php
public function insert($insert, $index)
    {
        return new static(StaticStringy::insert(
            $this->string, $this->retrieveValue($insert), $this->getIntegerable($index))
        );
    }
public function insert($insert, $index)
    {
        return new static(StaticStringy::insert(
            $this->string, $this->retrieveValue($insert), $this->getIntegerable($index))
        );
    }
Insert $insert into the String at the $index provided.

@param mixed $insert
@param mixed $index
@return static