ProrataTemporis

    function prorataTemporis($from = '2018-06-15', $to = '2018-08-01')
    {
        $from = new \DateTime($from);

        $to = new \DateTime($to);

        $diff = $to->diff($from);

        $months = $diff->y * 12 + $diff->m;

        return $months + (1 / $from->format('t') * $diff->d);
    }

    print $this->prorataTemporis('2016-06-01') . PHP_EOL;
        // 26
    
    print $this->prorataTemporis('2018-06-01') . PHP_EOL;
        // 2
    
    print $this->prorataTemporis('2018-07-15') . PHP_EOL;
        // 0.54838709677419
    
    print $this->prorataTemporis('2018-07-31') . PHP_EOL;
        // 0.032258064516129