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];
    }
}

categories[clothing[0]] = "some value"

$newArray=array_keys($originalArray);

$count = count($clothing);
for($i=0; $i<$count; $i++)
    $clothing[$i] = (array_key_exists($clothing[$i], $categories))
        ? $categories[$clothing[$i]] : 0;

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

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

array_walk($clothing,
           function(&$value) use($categories) {
               if (isset($categories[$value]))
                   $value = $categories[$value];
           }
);
var_dump($clothing);