json_encode之后,json数据变成[{"a""1"}],如何改成{"a""1"}

function get_js_array($sql){
	$result = mysql_query($sql);
	$arr = array();
	while($row = mysql_fetch_array($result, MYSQL_ASSOC)){
		$arr[] = $row;  
	}
	$js = json_encode($arr);
	mysql_free_result($result);
	mysql_close($conn);
	return $js;
}