- array_intersect_key: documentation ( source)
- print_r: documentation ( source)
- array_fill_keys: documentation ( source)
<?php
$o1 = array('foo' => '1', 'bar' => '2');
$o2 = array('foo' => '3', 'baz' => '4');
$o3 = array_intersect_key($o2, array_fill_keys(array('foo'), null));
print_r($o1 + $o3);