<?php

$categories['t-shirts'] = 10;
$categories['shorts'] = 11;

$clothing[0] = 't-shirts';
$clothing[1] = 'shorts';

foreach($clothing as $key => $val){
    if(isset($categories[$val])){
         $clothing[$key] = $categories[$val];
    }
}

print_r($clothing);

Array
(
    [0] => 10
    [1] => 11
)