[Ъь_]

Есть несколько решений, например вот

<?php
 function json_fix_cyr($json_str) {
     $cyr_chars = array (
         '\u0430' => 'а', '\u0410' => 'А',
         '\u0431' => 'б', '\u0411' => 'Б',
         '\u0432' => 'в', '\u0412' => 'В',
         '\u0433' => 'г', '\u0413' => 'Г',
         '\u0434' => 'д', '\u0414' => 'Д',
         '\u0435' => 'е', '\u0415' => 'Е',
         '\u0451' => 'ё', '\u0401' => 'Ё',
         '\u0436' => 'ж', '\u0416' => 'Ж',
         '\u0437' => 'з', '\u0417' => 'З',
         '\u0438' => 'и', '\u0418' => 'И',
         '\u0439' => 'й', '\u0419' => 'Й',
         '\u043a' => 'к', '\u041a' => 'К',
         '\u043b' => 'л', '\u041b' => 'Л',
         '\u043c' => 'м', '\u041c' => 'М',
         '\u043d' => 'н', '\u041d' => 'Н',
         '\u043e' => 'о', '\u041e' => 'О',
         '\u043f' => 'п', '\u041f' => 'П',
         '\u0440' => 'р', '\u0420' => 'Р',
         '\u0441' => 'с', '\u0421' => 'С',
         '\u0442' => 'т', '\u0422' => 'Т',
         '\u0443' => 'у', '\u0423' => 'У',
         '\u0444' => 'ф', '\u0424' => 'Ф',
         '\u0445' => 'х', '\u0425' => 'Х',
         '\u0446' => 'ц', '\u0426' => 'Ц',
         '\u0447' => 'ч', '\u0427' => 'Ч',
         '\u0448' => 'ш', '\u0428' => 'Ш',
         '\u0449' => 'щ', '\u0429' => 'Щ',
         '\u044a' => 'ъ', '\u042a' => 'Ъ',
         '\u044b' => 'ы', '\u042b' => 'Ы',
         '\u044c' => 'ь', '\u042c' => 'Ь',
         '\u044d' => 'э', '\u042d' => 'Э',
         '\u044e' => 'ю', '\u042e' => 'Ю',
         '\u044f' => 'я', '\u042f' => 'Я',
  
         '\r' => '',
         '\n' => '<br />',
         '\t' => ''
     );
  
     foreach ($cyr_chars as $cyr_char_key => $cyr_char) {
         $json_str = str_replace($cyr_char_key, $cyr_char, $json_str);
     }
     return $json_str;
 }
  
 echo json_fix_cyr(json_encode(array("собака","кошка"))); // ["собака","кошка"]
 ?>