Enter a string and print out all the permutations of the characters in the string in dictionary order. For example, if you input the string a B c, you print out all the strings a B c, a C b, B a c, B C a, C a B and C B a that can be arranged by the characters a,b,c. 
Train of thought:
1. Forming recursive tree by recursion to achieve the effect of depth first and fixed initials
 2. Depth first only after reduction
 3. Retrospective Thought
 4. A picture and a running process can only be realized slowly.

<?php
function test($str,$start,&$res){
        //Recursive Termination Conditions
        if($start==strlen($str)){
                $res[]=$str;
                return;
        }   
        //  
        for($i=$start;$i<strlen($str);++$i){
                if($i==$start || $str{$i}!=$str{$start}){
                        swap($str,$i,$start);var_dump($str.'==='.$start);var_dump($res);sleep(1);
                        test($str,$start+1,$res);
                        swap($str,$i,$start);
                }   
    
        }   
        return $res;
}
function swap(&$str,$a,$b){
        if(!is_string($str)) return;
        $t=$str{$a};
        $str{$a}=$str{$b};
        $str{$b}=$t;
}
$str="abc";
$res=array();
//Call entry,Start with index 0
$res=test($str,0,$res);
var_dump($res);

string(7) "abc===0"
array(0) {
}
string(7) "abc===1"
array(0) {
}
string(7) "abc===2"
array(0) {
}
string(7) "acb===1"
array(1) {
  [0]=>
  string(3) "abc"
}
string(7) "acb===2"
array(1) {
  [0]=>
  string(3) "abc"
}
string(7) "bac===0"
array(2) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
}
string(7) "bac===1"
array(2) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
}
string(7) "bac===2"
array(2) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
}
string(7) "bca===1"
array(3) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
}
string(7) "bca===2"
array(3) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
}
string(7) "cba===0"
array(4) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
}
string(7) "cba===1"
array(4) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
}
string(7) "cba===2"
array(4) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
}
string(7) "cab===1"
array(5) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
  [4]=>
  string(3) "cba"
}
string(7) "cab===2"
array(5) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
  [4]=>
  string(3) "cba"
}
array(6) {
  [0]=>
  string(3) "abc"
  [1]=>
  string(3) "acb"
  [2]=>
  string(3) "bac"
  [3]=>
  string(3) "bca"
  [4]=>
  string(3) "cba"
  [5]=>
  string(3) "cab"
}