foo.php

<?php

class TimePeriod
{
    protected $periodRepresentation;

    protected function __construct($periodRepresentation)
    {
        $this->periodRepresentation = $periodRepresentation;
    }

    public static function asMinutes($periodRepresentation)
    {
        return (new static($periodRepresentation))->parse();
    }

    public static function asHours($periodRepresentation)
    {
        $minutes = (new static($periodRepresentation))->parse();
        return round($minutes / 60, 2);
    }

    public static function asDays($periodRepresentation)
    {
        $minutes = (new static($periodRepresentation))->parse();
        return round(($minutes / 60) / 24, 2);
    }

    public static function asWeeks($periodRepresentation)
    {
        $minutes = (new static($periodRepresentation))->parse();
        return round((($minutes / 60) / 24) / 7, 2);
    }

    protected function toMinutes($amount, $type)
    {
        switch ($type) {
            case 'w':
                $expression = 7 * 24 * 60;
                break;
            case 'd':
                $expression = 24 * 60;
                break;
            case 'h':
                $expression = 60;
                break;
            case 'm':
                $expression = 1;
                break;
        }

        return $expression * $amount;
    }

    protected function parse()
    {
        $minutes = 0;
        $periodRepresentations = explode(' ', $this->periodRepresentation);
        foreach ($periodRepresentations as $periodRepresentation) {
            if (preg_match('/([0-9]*)(w|d|h|m){1}/', $periodRepresentation, $matches)) {
                $minutes += $this->toMinutes($matches[1], $matches[2]);
            }
        }

        return $minutes;
    }
}

echo TimePeriod::asMinutes('7d 12h 30m') . "\n";
echo TimePeriod::asHours('7d 12h 30m') . "\n";
echo TimePeriod::asDays('7d 12h 30m') . "\n";
echo TimePeriod::asWeeks('7d 12h 30m');