PHP按自定义顺序排序三个字母字符

I'm trying to sort groups of three characters like LMH, MMH, HHL etc all containing only the characters L, M and H but I need them sorted in the order LMH. This is what I have but not sure how to compare them. The $val array doesn't work when there are duplicate characters. It's a string that is broken into an array by the function.

function sortit($str) {
 $val = ['L' => 0, 'M' => 1, 'H' => 2];
 $parts = str_split($str);
 foreach ($parts as $value) {
   $order[$val[$value]] = $value;
 }
 ksort($order);
 return implode('',$parts);
 }

Input:

MLH 
HLL 
MHM 
LHM 
MLH 
LHM 
MHL

Desired output:

LMH 
LLH 
MMH 
LMH 
LMH 
LMH 
LMH

Use usort doc:

function sortit($str) {
   $val = ['L' => 0, 'M' => 1, 'H' => 2];
   $parts = str_split($str);
   usort($parts, function ($a, $b) use ($val) {return $val[$a] - $val[$b];});
   return implode("", $parts);
 }

Use it as:

echo sortit("MLH"); // output LMH 

$val = ['L' => 0, 'M' => 1, 'H' => 2];
$strings = ['HML', 'MHM', 'LHL'];

foreach ($strings as $str) {
    $arr = str_split($str);
    usort($arr, function($val1, $val2) use ($val) {
        return $val[$val1] <=> $val[$val2];
    });
    var_dump(implode('', $arr));
}

You can sort it with usort and a custom compare function. Here I used your val array, which was a good first idea.