<?php

$deposits = [
  'СБ' =>['RUR'=>['90'=>7.0, '180'=>9.1], 'USD'=>['90'=>3.2, '180' => 5.2]],
  'ВТБ'=>['RUR'=>['90'=>7.2, '180'=>9.2], 'USD'=>['90'=>3.1, '180' => 5.0]],
  'МКБ'=>['RUR'=>['90'=>7.1, '180'=>9.3], 'USD'=>['90'=>3.3, '180' => 5.1]],
];

function compareBy ($currency, $term) {
  return function ($bank1, $bank2) use ($currency, $term) {
     $value1 = $bank1[$currency][$term];
     $value2 = $bank2[$currency][$term];
     if ($value1 == $value2)
       return 0;
     else
       return $value1 < $value2 ? -1 : 1;
  };
}

function printArray ($array, $inline=false) {
  $i=0;
  foreach ($array as $key=>$value) {
    if ($inline && $i++) echo ',';
    echo "$key=>";
    if (is_array ($value)) { echo '['; printArray ($value, true); echo ']'; }
    else echo $value;
    if (!$inline) echo PHP_EOL;
  }
  if (!$inline) echo PHP_EOL;
}

uasort ($deposits, compareBy ('RUR',  '90')); printArray($deposits);
uasort ($deposits, compareBy ('USD', '180')); printArray($deposits);