array_reduce() can't work as associative-array "reducer" for PHP?

First, array_reduce() works with associative arrays, but you don't have any chance to access the key in the callback function, only the value.

You could use the use keyword to access the $result by reference in the closure like in the following example with array_walk(). This would be very similar to array_reduce():

$array = array(
    'foo' => 'bar',
    'hello' => 'world'
);

// Inject reference to `$result` into closure scope.
// $result will get initialized on its first usage.
array_walk($array, function($val, $key) use(&$result) {
    $result .= " $key=\"$val\"";
});
echo "<row$result />";

Btw, imo your original foreach solution looks elegant too. Also there will be no significant performance issues as long as the array stays at small to medium size.

$array = array(
    'foo' => 'bar',
    'hello' => 'world'
);

$OUT = join(" ", array_reduce(array_keys($array), function($as, $a) use ($array) {
    $as[] = sprintf('%s="%s"', $a, $array[$a]); return $as;
}, array()));

I personally see nothing wrong with the foreach thing, but if you want one single expression, your map snippet can be simplified down to

$OUT = sprintf("<row %s/>",
    join(" ", array_map(
        function($a, $b) { return "$a=\"$b\""; },
        array_keys($assoc),
        array_values($assoc)
)));

Also, since you're generating XML, it's better to use a dedicated tool, for example:

$doc = new SimpleXMLElement("<row/>");
foreach($assoc as $k => $v)
    $doc->addAttribute($k, $v);
echo $doc->asXML();