Return occurence of letters in order in the string

I made this code to return the number of occurrences of letters in a string. this function returns ‘6a3b5y’ but I want it to respect the order and display ‘4a3b2a5y’ how to change it to display ‘4a3b2a5y’ ?

function encode($input) {
    $l = mb_strlen($input, 'UTF-8');
    $unique = array();
    for($i = 0; $i < $l; $i++) {
        $char = mb_substr($input, $i, 1, 'UTF-8');
        if(!array_key_exists($char, $unique))
            $unique[$char] = 0;
        $unique[$char]++;
    }
    $a=[];
    foreach($unique as $k=>$val){
    $a[]= $val.$k ;
    }
      return implode("",$a) ;
}
echo encode('aaaabbbaayyyyy') ;

Answer

Use a variable to hold the previous character. Compare the current character to it. If they’re the same, increment a counter. If they’re not, output the previous character and counter, then reset the variables.

At the end, output the final character and counter.

function encode($input) {
    $output = '';
    $l = mb_strlen($input, 'UTF-8');
    $prevchar = mb_substr($input, 0, 1, 'UTF-8');
    $count = 1;
    for($i = 1; $i < $l; $i++) {
        $char = mb_substr($input, $i, 1, 'UTF-8');
        if ($char == $prevchar) {
            $count++;
        } else {
            $output .= "$count$prevchar";
            $count = 1;
            $prevchar = $char;
        }
    }
    $output .= "$count$prevchar";
    return $output;
}