Fewest Coins Problem in PHP

Question

Can someone hekp me please solve this problem?
Given a string comprised of lowercase letters in the range ascii[a-z],where each letter represents a coin type, determine the length of the shoortest substring that contains at least one of each type of coin.
Example:
coins: dabbcabcd
The list of all caracters in the string is [a,b,c,d]
two of the substrings that contains all letters are dabbc and abcd.
the shortest substring that contains all of the letters is 4 characters long.

Answer 1

I have tried this but it only gives me the length of the distinct characters in the string
function fewestCoins($coins){

$count=0;
$strArray = count_chars($coins,1);
foreach ( $strArray as $key=>$value){
if ($value>0) {
$count++;
}
}
echo($count);
$testarr=substr_count($coins,"abcd");
echo($testarr);
}