This is dangerous:
//example array
$array = array('a' => array(array(243,453,435,232)));
foreach ($array['a'] as &$value) {
$value[3] = $newvalue;
}
This works, but now the $value var is in the array by reference which can lateron lead to:
Notice: Array to string conversion
EDIT:
see below in the comments for when…
This works better: