$arr = [300, 600, 900, 1200];
$number = 920;
$deltas = array_reduce(
    $arr, 
    function($t, $v) use ($number) { 
        $t[$v] = abs($v - $number); 
        return $t; 
    },
    []
);
asort($deltas);
echo array_keys($deltas)[0];