使用转义斜杠时出现JSON语法错误

TOTALLY RE-EDITED QUESTION

完全重新编辑的问题

<?php
   $s1 = '{"req":"auth","token":"1234567\/\"}'; // this seems to be valid, isn't it?
   $s2 = '{"req":"auth","token":"123456\/\7"}'; // this seems to be valid, isn't it?
   $s3 = '{"req":"auth","token":"123456789"}';

   print_r(json_decode($s1,true));
   echo " - ERRORCODE FOR CASE#1 IS:" . json_last_error() . '<br />';

   print_r(json_decode($s2,true));
   echo " - ERRORCODE FOR CASE#2 IS:" . json_last_error() . '<br />';

   print_r(json_decode($s3,true));
   echo " - ERRORCODE FOR CASE#3 IS:" . json_last_error() . '<br />';
?>
<?php