Capitalize random letters of a string in PHP

one text

Solution:

Let's assume you want to CAPITALIZE 5 letters randomly:

$string = "hellohg.09ui8vkosjbdh";
$characters = str_split($string);
$i = 0;
do{
    $random_index = rand(0, count($characters) - 1);
    $unique_indices[] = ""; //UNIQUE INDICES
    while (in_array($random_index, $unique_indices)) {
        $random_index = rand(0, count($characters) - 1);
    }
    $unique_indices[] = $random_index;

    $random_letter = $characters[$random_index];
    if(ctype_alpha($random_letter)){//only letters 
        $characters[$random_index] = strtoupper($random_letter);
        $i++;
    }
}while($i<5);echo implode('', $characters);

Thanks to @El_Vanja for Note UNIQUE INDICES