php函数json_encode兼容低版本

if(!function_exists('json_encode')) {
function json_encode($arg) {
   $returnValue = '';
   $c           = '';
   $i           = '';
   $l           = '';
   $s           = '';
   $v           = '';
   $numeric     = true;

   switch (gettype($arg)) {
    case 'array':
     foreach ($arg AS $i => $v) {
      if (!is_numeric($i)) {
       $numeric = false;
       break;
      }
     }

     if ($numeric) {
      foreach ($arg AS $i => $v) {
       if (strlen($s) > 0) {
        $s .= ',';
       }
       $s .= json_encode($arg[$i]);
      }

      $returnValue = '[' . $s . ']';
     } else {
      foreach ($arg AS $i => $v) {
       if (strlen($s) > 0) {
        $s .= ',';
       }
       $s .= json_encode($i) . ':' . json_encode($arg[$i]);
      }

      $returnValue = '{' . $s . '}';
     }
     break;

    case 'object':
     foreach (get_object_vars($arg) AS $i => $v) {
      $v = json_encode($v);

      if (strlen($s) > 0) {
       $s .= ',';
      }
      $s .= json_encode($i) . ':' . $v;
     }

     $returnValue = '{' . $s . '}';
     break;

    case 'integer':
    case 'double':
     $returnValue = is_numeric($arg) ? (string) $arg : 'null';
     break;

    case 'string':
     $returnValue = '"' . strtr($arg, array(
      " "   => '\r',    " "   => '\n',    "	"   => '\t',     ""   => '\b',
      "f"   => '\f',    '\'   => '\\',   '"'    => '"',
      "x00" => 'u0000', "x01" => 'u0001', "x02" => 'u0002', "x03" => 'u0003',
      "x04" => 'u0004', "x05" => 'u0005', "x06" => 'u0006', "x07" => 'u0007',
      "x08" => '',     "x0b" => 'u000b', "x0c" => 'f',     "x0e" => 'u000e',
      "x0f" => 'u000f', "x10" => 'u0010', "x11" => 'u0011', "x12" => 'u0012',
      "x13" => 'u0013', "x14" => 'u0014', "x15" => 'u0015', "x16" => 'u0016',
      "x17" => 'u0017', "x18" => 'u0018', "x19" => 'u0019', "x1a" => 'u001a',
      "x1b" => 'u001b', "x1c" => 'u001c', "x1d" => 'u001d', "x1e" => 'u001e',
      "x1f" => 'u001f'
     )) . '"';
     break;

    case 'boolean':
     $returnValue = $arg?'true':'false';
     break;

    default:
     $returnValue = 'null';
   }

   return $returnValue;
}
}