Better version at https://github.com/microformats/microformats-parser-website-php/blob/master/json-validator.php

is_valid_mf2_json.php

<?php

function is_valid_mf2_json($input) {
	// Input to this function must be an array
	if(!is_array($input))
		return false;

	// Keys type and properties are required at a minimum and must be arrays
	if(!isset($input['type']) || !is_array($input['type']))
		return false;
	if(!isset($input['properties']) || !is_array($input['properties']))
		return false;

	// Every value of type must be a string beginning with h-
	foreach($input['type'] as $type) {
		if(!is_string($type) || substr($type, 0, 2) != 'h-')
			return false;
	}

	foreach($input['properties'] as $property) {
		// Every property must be an array
		if(!is_array($property))
			return false;

		// If a value of a property is not a string, it must be a valid mf2 object
		foreach($property as $val) {
			if(!is_string($val)) {
				if(!is_valid_mf2_json($val))
					return false;
			}
		}
	}

	return true;
}