php usort two columns

1$users=array(                                                       
2    array("sex"=>0,"age"=>9),                              
3    array("sex"=>0,"age"=>34),                             
4    array("sex"=>1,"age"=>17),                             
5    array("sex"=>0,"age"=>1),                              
6    array("sex"=>1,"age"=>3)                              
7);                                                                        
8//sort first by sex, then by age
9usort($users, function($a,$b){                                      
10   return  ($a['sex'] == $b['sex']) ?                   
11            $a['age'] < $b['age'] :                                   
12            $a['sex'] < $b['sex'];                      
13});