How to generate all permutations of a string in PHP?

You can use a back tracking based approach to systematically generate all the permutations:

// function to generate and print all N! permutations of $str. (N = strlen($str)).
function permute($str,$i,$n) {
   if ($i == $n)
       print "$str\n";
   else {
        for ($j = $i; $j < $n; $j++) {
          swap($str,$i,$j);
          permute($str, $i+1, $n);
          swap($str,$i,$j); // backtrack.
       }
   }
}

// function to swap the char at pos $i and $j of $str.
function swap(&$str,$i,$j) {
    $temp = $str[$i];
    $str[$i] = $str[$j];
    $str[$j] = $temp;
}   

$str = "hey";
permute($str,0,strlen($str)); // call the function.

Output:

#php a.php
hey
hye
ehy
eyh
yeh
yhe

My variant (works as well with array or string input)

function permute($arg) {
    $array = is_string($arg) ? str_split($arg) : $arg;
    if(1 === count($array))
        return $array;
    $result = array();
    foreach($array as $key => $item)
        foreach(permute(array_diff_key($array, array($key => $item))) as $p)
            $result[] = $item . $p;
    return $result;
}

P.S.: Downvoter, please explain your position. This code uses additional str_split and array_diff_key standard functions, but this code snippet is the smallest, it implements pure tail recursion with just one input parameter and it is isomorphic to the input data type.

Maybe it will lose benchmarks a little when comparing with other implementations (but performance is actually almost the same as in @codaddict's answer for several character strings), but why we can't we just consider it as one of the different alternatives which has its own advantages?

How to generate all permutations of a string in PHP?

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