Sort characters in string giving priority by lower-case, then upper-case, then numbers then symbols

Question

I am new to PHP and trying to do programming questions that involve sorting. I want to solve this problem in which the input and output are as followed.

Before Sort: 6Ata7~58jh&*(*HLy^)&a[yjJ>8]G8,$9<Y;B:%^

After Sort: jhytaayjYBJGAHL8968758~&*(*^)&[>],$<;:%^

I need help understanding how to sort according to the order state mention in the title and how I can get the same result from the above input and output. Below is the code I'm working on:

function reverseString($string){
    $stringlen = strlen($string) - 1;
    $split_str = str_split($string, 1);

    $array = array();

    for($i = 0; $i <= $stringlen; $i++){
        array_push($array, $split_str[$i]);
    }

    rsort($array, SORT_STRING | SORT_NUMERIC | SORT_FLAG_CASE | SORT_NATURAL);

    return $array;
}

if(isset($_POST['input']) && !empty($_POST['input'])){
    echo '<pre>';
    echo 'Before sort: ';
    print_r($_POST['input']);
    echo '<br>--------------------<br>';
    echo 'After sort:' ;
    print_r(reverseString($_POST['input'])); 
    echo '</pre>';
}

Answer 1

One option would be to sort the array of string characters using a lambda function. We can use usort() on the array, with a helper function get_rank() returning a value, from 1 to 4, depending on whether the character be lowercase, uppercase, numeric, or anything else (including symbols).

function get_rank($letter) {
    if (ctype_lower($letter)) {
        return 1;
    }
    else if (ctype_upper($letter)) {
        return 2;
    }
    else if (is_numeric($letter)) {
        return 3;    
    }
    else {
        return 4;
    }
}

$input = "6Ata7~58jh&*(*HLy^)&a[yjJ>8]G8,$9<Y;B:%^";
echo $input . "\n";
$letters = str_split($input);
usort($letters, function($a, $b) {
    return get_rank($a) > get_rank($b);
});
$output = implode("", $letters);
echo $output;

This prints:

6Ata7~58jh&*(*HLy^)&a[yjJ>8]G8,$9<Y;B:%^
ajyjhatyYBAGJHL9886875,%:;~<$&)&]*>(*^[^

Note that you didn't specify any logic for sorting within groups of lowercase, symbols, etc. The above logic can be modified to include this.

Answer 2

PHP does not have any native functions that sort strings by their characters so the string must be split before implementing a sorting algorithm.

One technique is to use specifically ordered optional capture groups in a preg_match_all() call then use its results to sort the split array. (Like this answer). This has the added benefit of sorting in an ascending fashion within each character category.

Code: (Demo)

preg_match_all('/([a-z])|([A-Z])|(\d)|(.)/', $chars, $m, PREG_SET_ORDER);
$array = str_split($chars);
array_multisort($m, $array);
echo implode($array);

---

To preserve the original order of characters within their respective categories, leverage a lookup array to determine the priority of encountered characters. (Like this answer)

Code: (Demo)

$lookup = array_fill_keys(range('a', 'z'), 0) + array_fill_keys(range('A', 'Z'), 1) + array_fill_keys(range(0, 9), 2);
$array = str_split($chars);
usort($array, fn($a, $b) => ($lookup[$a] ?? 3) <=> ($lookup[$b] ?? 3));
echo implode($array);