recursive-array-modification.php

<?php
$data = array('foo', '', 'bar');

array_walk_recursive($data, function (&$value, $key) {
    if ($value === '') {
        $value = null;
    }
});

var_dump($data);

// array(3) { [0]=> string(3) "foo" [1]=> NULL [2]=> string(3) "bar" }