Решение 6 задачи ( самый грамотный способ с применением статических функций)

<?php
class Point
{
    public $x;
    public $y;
    function __construct($x, $y)
    {
        $this->x = $x;
        $this->y = $y;
    }
    function distance(Point $point)
    {
        return sqrt(pow($this-> x -  $point-> x,2 )+pow( $this-> y  - $point-> y, 2));
    }
}

class Trapezoid
{
    public $AB;
    public $BC;
    public $CD;
    public $DA;
    public $h;
    public $perimetr;
    public $s;
//
    public function __construct($x1, $y1, $x2, $y2, $x3, $y3, $x4, $y4)
    {
        $A = new Point($x1, $y1);
        $B = new Point($x2, $y2);
        $C = new Point($x3, $y3);
        $D = new Point($x4, $y4);
        $this->AB = $A->distance($B);
        $this->BC = $B->distance($C);
        $this->CD = $C->distance($D);
        $this->DA = $D->distance($A);
        $this->h = sqrt(pow($this->AB,2)-pow(($this->DA - $this->BC )/2,2));
    }
    function isEquilateral() : bool
    {
        return $this->AB == $this->CD;
    }
    function getSquare()
    {
        $this->s = ($this->BC+$this->DA) / 2 * $this->h;
        return $this->s;
    }
    function getPerimeter()
    {
        $this->perimetr = $this->AB + $this->BC + $this->CD + $this->DA;
        return $this->perimetr;
    }
}
class MethodsOfTrapezoid
{
    public static $sumSquare;
    public static $averSquare;
    public static $quantity = 0;
    public static function CreateTrapezoids($n)
    {
        for ($i = 0; $i < $n; $i++)
        {
            $trapecia[$i] = new Trapezoid(rand(5, 10), rand(5, 10), rand(5, 10), rand(5, 10), rand(5, 10), rand(5, 10), rand(5, 10), rand(5, 10));
            self::$sumSquare += $trapecia[$i]->getSquare();
        }
        return $trapecia;
    }
    public static function QuantityCalc($n, $trapecia)
    {
        self::$averSquare = self::$sumSquare / $n;
        for ($i = 0 ; $i < $n; $i++)
        {
            if($trapecia[$i]->getSquare() > self::$averSquare)
            {
                self::$quantity++;
            }
        }
    }
    public static function Show($n,$trapecia)
    {
        for($i = 0; $i < $n; $i++)
        {
            echo "Square of trapezoid " . $i ." : ".$trapecia[$i]->getSquare() . "\n<BR>";
            echo "Perimeter of trapezoid " . $i ." : ".$trapecia[$i]->getPerimeter() . "\n<BR>";
            if ($trapecia[$i]->isEquilateral())
            {
                echo "trapezoid " . $i . " is equilateral \n<BR>";
            }
            else {

                echo "trapezoid " . $i . " is UNequilateral \n<BR>";
            }
        }
        echo "average Square: ".self::$averSquare . "\n<BR>";
        echo "Quantity of trapezoids which square more then average Square: ".self::$quantity . "\n<BR>";    
    }
}
$massTrapezoid = MethodsOfTrapezoid::CreateTrapezoids(4);
MethodsOfTrapezoid::QuantityCalc(4, $massTrapezoid);
MethodsOfTrapezoid::Show(4, $massTrapezoid);