php to reverse a string without using loops or builtin functions

How would you write a short php function to reverse a string. The function must:

• have only one argument
• not use the built-in function 'st开发者_如何学Crrev' or 'array_reverse'
• not a use a looping construct like 'for', 'foreach' or 'while'.

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Quickly scanning down, these all look so long!

function rev($str) {
    return $str?rev(substr($str,1)).$str[0]:'';
}

Recursive so obviously doesn't work on strings longer than 100 chars.

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Since this sounds like homework question I'll tell you how but you code it yourself. Turn the string into an array with a cast. Then use one of the array sorting functions that takes a user defined sorting function.

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function reverseString($string) {
  return shell_exec(sprintf('ruby -e \'puts "%s".reverse\'', preg_replace("/\"/", "\\\"", $string)));
}

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Recursive solution:

function sr( $txt ){
    return $txt[ strlen( $txt ) - 1 ] . ( ( strlen( $txt ) > 1 )?sr( substr( $txt, 0, strlen($txt)-1 ) ):null) ;
}

echo sr( "abc def ghi jklm" );

Explanation:

return $txt[ strlen( $txt ) - 1 ] // return last byte of string
.                                 // concatenate it with:
(( strlen( $txt ) > 1 ) ?         // if there are more bytes in string
 sr( substr( $txt, 0, strlen( $txt ) - 1 ) // then with reversed string without last letter
 : null );                        // otherwise with null

To make it work with zero-length string, another conditional expression was added:

return (strlen($txt))? ($txt[ strlen( $txt ) - 1 ] . ( ( strlen( $txt ) > 1 )?sr( substr( $txt, 0, strlen($txt)-1 ) ):null)):"" ;

---

    <?php
    // Reversed string and Number
    //  For Example :
        $str = "hello world. This is john duvey";
        $number = 123456789;
        $newStr = strrev($str);
        $newBum = strrev($number);

        echo $newStr;
        echo "<br />";
        echo $newBum;

OUTPUT : 
 first : yevud nhoj si sihT .dlrow olleh
 second: 987654321`enter code here`

---

Meets all your requirements but works only in PHP 5.3 or higher. Making it work on others is left as homework.

function reverse($str) {
        $i=0;
        $j=strlen($str)-1;
start:
        if($i>=$j) {
                goto done;
        }
        $tmp = $str[$j];
        $str[$j--] = $str[$i];
        $str[$i++] = $tmp;
        goto start;
done:
        return $str;
}   

---

function reverse_string($string) {
    if($string !== '')
        return substr($string, -1).reverse_string(substr($string, 0, strlen($string)-2));     
}

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A recursive solution. Feels like something like that is what your teacher is looking for.

function my_strrev ($str) {
  $length = strlen($str);
  switch ($length) {
      case 0: return '';
      case 1: return $str; break;
      case 2: return $str[1] . $str[0];
      default :
          return $str[$length-1] .  my_strrev(substr($str,1,-1)) . $str[0];
          break;
  }
}

It swaps the first and the last letter and than makes the same with the rest of the string.

Update: Inspired by mateusza I created another solution (its fun ;))

function my_strrev2 ($str) {
  return $str
    ? my_strrev2(substr($str, 1)) . $str[0]
    : '';
}

It works similar to mateuszas, but this one appends the first character, instead of prepend the last one.

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My answer is OOP and uses recursion. Takes care by itself of the recursion limit.

class StringReverser
{
    public $reversed_string;
    public function __construct ($string) {
        $original_recursion_limit = ini_get('pcre.recursion_limit');
        $array = str_split($string);
        krsort($array);
        $i = strlen($string);
        ini_set('pcre.recursion_limit', $i+1);
        $this->add2string($string, $i, $array);
        ini_set('pcre.recursion_limit', $original_recursion_limit);
    }

    public function reverse() {
        return $this->reversed_string;
    }

    private function add2string ($s, $i,  $a) {
        if($i) {
            $i--;
            $this->reversed_string .= $a[$i];
            $this->add2string($s, $i,$a, $this->reversed_string);
        }
    }
}


$string =  "Elzo Valugi";
echo $string ."<br>";
$reverser = new StringReverser($string);
echo $reverser->reverse();

---

<?php
 $string="jomon is name my";
 for($i=strlen($string);$i>=0;$i--)
 {
     $char.=$string{$i};    
 }
 echo $char."<br/>";

 for($i=0; $i<strlen($char);$i++)
 {
     if($char[$i+1]==" " || $char[$i+1]=="")
     {
         for($temp=$i; $temp>=0 && $char[$temp]!=' '; $temp--)
             echo $char[$temp];
     }
     echo " ";
 }    
 ?>