[suspicious - maybe spam] [suspicious - maybe spam] [PHP] how to get checkbox form data as array?

["tx" <tx@xxxxxxxxxx>]

i create the html form as below

<input type=checkbox name=aaa value=1>
<input type=checkbox name=aaa value=2>
<input type=checkbox name=aaa value=3>
<input type=checkbox name=aaa value=4>

and i receive the form data in php file as below:

global $aaa;
print_r($aaa);

this action only echo "4" not the total array in array format,why?