<?php declare(strict_types = 1);

class HelloWorld implements JsonSerializable
{
	private string $foo;
	
	public function __construct(string $foo) {
		$this->foo = $foo;
	}
	
	public function jsonSerialize() {
		return get_object_vars($this);
	}
}

// Usage
$foo = new HelloWorld('fizzbuzz');
echo json_encode($foo); // prints '{"foo":"fizzbuzz"}'