<!-- Collection of code snippets by Arne VajhÃ¸j -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
function all_in($needle, $haystack, $key) {
    return count(array_uintersect($needle, $haystack, function($v1, $v2) use($key) { return $v1[$key] == $v2[$key] ? 0 : 1; })) == count($needle);
}

function test($needle, $haystack) {
    if(all_in($needle, $haystack, 'by')) {
        echo "Found\r\n";
    } else {
        echo "Not found\r\n";
    }   
}

$haystack = array(array('id' => 1, 'navn' => 'Niels', 'by' => 'Århus'), array('id' => 2, 'navn' => 'Martin', 'by' => 'Odense'), array ('id' => 3, 'navn' => 'Christian', 'by' => 'Herning'));
$needle1 = array(array ('by' => 'Århus'), array ('by' => 'Odense'));
$needle2 = array(array ('by' => 'Århus'), array ('by' => 'Esbjerg'));

test($needle1, $haystack);
test($needle2, $haystack);
?>