Index.php

<?php


class House {
    // color of the house
    private $color;
    // only these colors are allowed
    private $allowedColors = [
        'black', 'blue', 'red', 'green'
    ];
    public function setColor($color) {
        // Black to black (lowercase)
        $color = strtolower($color);
        if ( in_array( $color, $this -> allowedColors ) ) {
            // if $color is in the $allowedColors array
            // we can set the color property
            $this -> color = $color;

        }
    }
    public function getColor() {
        if ($this -> color) {
            // if color is set
            return $this -> color;
        } else {
            // show an error message
            return 'No color is set. May be you have set a color which is not allowed';
        }
    }
}

// Example 1
$house1 = new House();
$house1 -> setColor('black');
echo $house1 -> getColor();

echo '<br>'; // a HTML line break to make it readable

$house2 = new House();
$house2 -> setColor('yellow'); // a not allowed color
echo $house2 -> getColor();