php - Reverse string without strrev -

some time ago during job interview got task reverse string in php without using strrev.

my first solution this:

$s = 'abcdefg'; $temp = ''; ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {     $temp .= $s{$length - $i - 1}; } var_dump($temp); // outputs string(7) "gfedcba" 

then asked me if without doubling memory usage (not using $temp variable or variable copy reversed string to) , failed. kept bugging me , since tried solve multiple times failed.

my latest try looks this:

$s = 'abcdefg'; ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {     $s = $s{$i * 2} . $s; } var_dump($s); // outputs string(14) "gfedcbaabcdefg" 

it's not solution chop off "abcdefg" after loop because still double amount of memory used. need remove last character in every iteration of loop.

i tried use mb_substr this:

$s = 'abcdefg'; ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {     $s = $s{$i * 2} . mb_substr($s, $length - $i - 1, 1); } var_dump($s); 

but gives me uninitialized string offset errors.

this i'm stuck (again). tried googling solutions found either echo characters directly or use temporary variable.

i found question php string reversal without using memory there's no answer fits needs.

that's interesting one. here's came with:

$s = 'abcdefghijklm'; for($i=strlen($s)-1, $j=0; $j<$i; $i--, $j++) {     list($s[$j], $s[$i]) = array($s[$i], $s[$j]); } echo $s; 

list() can used assign list of variables in 1 operation. doing swapping characters (starting first , last, second-first , second-last , on, till reaches middle of string)

output mlkjihgfedcba. not using other variables $s , counters, hope fits criteria.