php, 52 bytes

Result

WA

Code [DL]

<?= sort($a=str_split(readline()));echo implode($a);

stdin

5022428804995019663926491050987697423675350135304328895424060554691109806491736287360322362480645109

stdout

15022428804995019663926491050987697423675350135304328895424060554691109806491736287360322362480645109

stderr

PHP Notice:  Only variables should be passed by reference in /volume/CODE on line 1

strace