public function getStartDepth(): int
    {
        $startDepth = PHP_INT_MAX;

        foreach ($this->items as $item) {
            $startDepth = min($startDepth, $item->getZ());
        }

        return $startDepth;
    }
public function getStartDepth(): int
    {
        $startDepth = PHP_INT_MAX;

        foreach ($this->items as $item) {
            $startDepth = min($startDepth, $item->getZ());
        }

        return $startDepth;
    }
public function getStartDepth ( ) : int { $ startDepth = PHP_INT_MAX ; foreach ( $ this -> items as $ item ) { $ startDepth = min ( $ startDepth , $ item -> getZ ( ) ) ; } return $ startDepth ; }
Calculate start depth of this layer.

@return int mm
Calculate start depth of this layer .