<?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();