Very, very basic Circle object.

Circle.php

<?php
class Circle {
    private $_center;
    private $_radius;

    public function __construct(Point $center, $radius) {
        $this->_center = $center;
        $this->_radius = $radius;
    }

    /**
     * Checks if a point is inside this circle.
     * Credit: http://stackoverflow.com/a/7227057/1365831
     *
     * @param  Point $point
     * @return bool
     */
    public function isPointInside(Point $point) {
        $dx = abs($point->x - $this->_center->x);
        $dy = abs($point->y - $this->_center->y);

        if($dx + $dy <= $this->_radius)
            return true;

        if($dx > $this->_radius)
            return false;

        if($dy > $this->_radius)
            return false;

        if($dx * $dx + $dy * $dy <= $this->_radius * $this->_radius)
            return true;

        return false;
    }
}

Point.php

<?php
class Point {
    public $x;
    public $y;

    public function __construct($x, $y) {
        $this->x = $x;
        $this->y = $y;
    }
}