Follow your json template. The result of json_decode will be like this
array (
'customers' =>
array (
0 =>
array (
'id' => 1123,
'email' => '<eMail>',
),
),
)
So, you should use foreach loop, or you can get the value with specify key.
eg:
$customer_array['customers'][0]['id']
$customer_array['customers'][0]['email']