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;
}