merge.php

<?php

$options = array(
	'conditions' => array(
		'type' => 'json',
		'accept' => array('application/json'),
	),
);


$options = array_replace_recursive(array(
	'conditions' => array(
		'type' => null,
		'accept' => array(),
	),
	'encode' => null,
), $options);

print_r(compact('options'));
/**
Array
(
    [options] => Array
        (
            [conditions] => Array
                (
                    [type] => json
                    [accept] => Array
                        (
                            [0] => application/json
                        )

                )

            [encode] =>
        )

)
*/