How to change random key + secret values in php?

How to change random key + secret values in php?

I have the following values:

key1: 111111111
secret1: 1111111111111111111111111 

key2: 222222222
secret2: 2222222222222222222222222

How to change random key + secret

$access_key = '111111111';
$access_secret = '1111111111111111111111111';

or

$access_key = '222222222';
$access_secret = '2222222222222222222222222';

Note: not show

$access_key = '1111111111';
$access_secret = '2222222222222222222222222';

Thanks for everyone's help !