php5.2以下版本json_encode兼容性
php5.1版本通过编译加载的json.so，在使用json_encode时会把键名为空的数组
$arr =array(''=>array('id'=>'aaa','total'=>'bbb'))转化成空；但是在php5.2中使用使用自带的此函数不存在此问题。
或者不想编译加载json.so扩展，可以使用替代函数
$jsonencode = version_compare(PHP_VERSION, '5.2.5', '>=') ? json_encode($arr) : php_json_encode($arr);echo $jsonencode;function php_json_encode($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($arg, array( "\r" => '\\r', "\n" => '\\n', "\t" => '\\t', "\b" => '\\b', "\f" => '\\f', '\\' => '\\\\', '"' => '\"', "\x00" => '\u0000', "\x01" => '\u0001', "\x02" => '\u0002', "\x03" => '\u0003', "\x04" => '\u0004', "\x05" => '\u0005', "\x06" => '\u0006', "\x07" => '\u0007', "\x08" => '\b', "\x0b" => '\u000b', "\x0c" => '\f', "\x0e" => '\u000e', "\x0f" => '\u000f', "\x10" => '\u0010', "\x11" => '\u0011', "\x12" => '\u0012', "\x13" => '\u0013', "\x14" => '\u0014', "\x15" => '\u0015', "\x16" => '\u0016', "\x17" => '\u0017', "\x18" => '\u0018', "\x19" => '\u0019', "\x1a" => '\u001a', "\x1b" => '\u001b', "\x1c" => '\u001c', "\x1d" => '\u001d', "\x1e" => '\u001e', "\x1f" => '\u001f' )) . '"'; break;case 'boolean':$returnValue = $arg?'true':'false';break;default:$returnValue = 'null';}return $returnValue;}