Palindrome

palindrome.php

<?php
//
// Testing on: 
// PHP 5.6.10-1+deb.sury.org~vivid+1 (cli)
//
function palindrome($string)
{
    ksort($counter = count_chars($string, 1));

    $ret = '';
    $mid = '';
    $hasOdd = 0;

    foreach ($counter as $char => $count) {
        if ($count&1) {
            if (++$hasOdd > 1) {
                return 'imp';
            }
            $mid = str_repeat(chr($char), $count);
        } else {
            $ret .= str_repeat(chr($char), $count/2);
        }
    }
    return sprintf('%s%s%s', $ret, $mid, strrev($ret));
}

assert('imp' == palindrome('abcd'));
assert('cddc' == palindrome('cdcd'));
assert('ccddaddcc' == palindrome('daccdccdd'));
assert('!3?ABabbaBA?3!' == palindrome('AABBaabb33!!??'));
assert('!3?ABabSSSbaBA?3!' == palindrome('AASSSBBaabb33!!??'));