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?

I would put all the characters in an array, and write a recursive function that will 'stripe out' all the remaining characters. If the array is empty, to a reference passed array.

<?php

$input = "hey";

function string_getpermutations($prefix, $characters, &$permutations)
{
    if (count($characters) == 1)
        $permutations[] = $prefix . array_pop($characters);
    else
    {
        for ($i = 0; $i < count($characters); $i++)
        {
            $tmp = $characters;
            unset($tmp[$i]);

            string_getpermutations($prefix . $characters[$i], array_values($tmp), $permutations);
        }
    }
}
$characters = array();
for ($i = 0; $i < strlen($input); $i++)
    $characters[] = $input[$i];
$permutations = array();

print_r($characters);
string_getpermutations("", $characters, $permutations);

print_r($permutations);

Prints out:

Array
(
    [0] => h
    [1] => e
    [2] => y
)
Array
(
    [0] => hey
    [1] => hye
    [2] => ehy
    [3] => eyh
    [4] => yhe
    [5] => yeh
)

Ah yes, combinations = order doens't matter. permutations = order does matter.

So hey, hye yeh are all the same combination, but 3 separate permutations as mentioned. Watch out that the scale of items goes up very fast. It's called factorial, and is written like 6! = 6*5*4*3*2*1 = 720 items (for a 6 character string). A 10 character string will be 10! = 3628800 permutations already, which is a very big array. In this example it's 3! = 3*2*1 = 6.

My approach uses recursion and no loops, please check and give feedback:

function permute($str,$index=0,$count=0)
{
    if($count == strlen($str)-$index)
        return;

    $str = rotate($str,$index);

    if($index==strlen($str)-2)//reached to the end, print it
    {
        echo $str."<br> ";//or keep it in an array
    }

    permute($str,$index+1);//rotate its children

    permute($str,$index,$count+1);//rotate itself
}

function rotate($str,$index)
{
    $tmp = $str[$index];
    $i=$index;
    for($i=$index+1;$i<strlen($str);$i++)
    {
        $str[$i-1] = $str[$i];
    }
    $str[$i-1] = $tmp;
    return $str;
}
permute("hey");

I made a simple class that uses Generators to create the permutations.This way you can just iterate over all possible combinations without exhausting the memory.

The class can take either a string or an array, and returns a Generator object which can be iterated over with foreach.

Obviously the longer the string or array, the longer it takes to generate all the permutations.

This has been build against PHP 7.4

class Permutation {

    /** @var string|array **/
    protected $permutationRoot;
    protected int $permutationLength;

    /**
     * @param $permutationRoot
     */
    protected function __construct( $permutationRoot ) {
        $this->permutationRoot = $permutationRoot;
        $this->permutationLength = is_array($permutationRoot)
            ? count($permutationRoot)
            : strlen($permutationRoot);
    }

    /**
     * @param string|array $permutationRoot
     *
     * @return \Generator
     */
    public static function resolve( $permutationRoot ): \Generator
    {
        $instance = new static($permutationRoot);

        return $instance->backTrack(
            $instance->permutationRoot,
            0,
             $instance->permutationLength,
         );
    }

    /**
     * @param string|array $permutation
     * @param int $index
     * @param int $length
     *
     * @return \Generator
     */
    protected function backTrack($permutation, int $index, int $length): \Generator
    {
        if ($index === $length) {
            yield $permutation;
        }

        for ($i = $index; $i < $length; $i++) {
            $this->swap($permutation, $index, $i);
            yield from $this->backTrack($permutation, $index + 1, $length);
            $this->swap($permutation, $index, $i); // backtrack.
        }
    }

    /**
     * @param $permutation
     * @param int $index
     * @param int $n
     *
     * @return void
     */
    protected function swap(&$permutation, int $index, int $n): void {
        $temp = $permutation[$index];
        $permutation[$index] = $permutation[$n];
        $permutation[$n] = $temp;
    }
}

// Test
foreach ( Permutation::resolve('hey') as $perm ) {
    echo $perm . "\n";
}

$sentence = "This is a cat";
$words = explode(" ", $sentence);
$num_words = count($words);
$uniqueWords = [];
for ($i = 0; $i < $num_words; $i++) {
    for ($j = $i; $j < $num_words; $j++) {
        $uniqueWord = '';
        for ($k = $i; $k <= $j; $k++) {
            $uniqueWord .= $words[$k] . ' ';
        }
        $uniqueWords[] = trim($uniqueWord);
    }
}

var_dump($uniqueWords);

This worked for me