I can make an associative array with keys but without values to fill them after in PHP

To complement the answer of Federico Saenz that is correct, I will detail a little about the use of arrangements.

First of all, depending on the version of PHP you are using, you can define an arrangement like this:

$arreglo = array('patron'=>,'usuario'=>);

or so from PHP 5.4

$arreglo = ['patron'=>,'usuario'=>];

Now once you have defined you can check the value of the keys created

var_dump($arreglo['patron']) //Retorna NULL
var_dump($arreglo['usuario']) //Retorna NULL

You can also modify the value assigned to the keys

$arreglo['patron'] = 666;
$arreglo['usuario'] = 'NekoOS';

I have even added more associations

$arreglo['nuevo'] = 'dato insertado previamente';

You could print the entire arrangement on the screen

print_r($arreglo['usuario']);
/** 
Tendrías un resultado así:

    Array
    (
        [patron] => 666
        [usuario] => NekoOS
        [nuevo] => dato insertado previamente
    )
*/

You can find more information about the use of arrangements and functions to operate with fixes in the official PHP page, here you have two links that could be useful:

• link
• link

Yes, you can. But remember to put the quotes to the keys so that they are of the string type.

$prueba = array('patron'=>'','usuario'=>'');

Greetings.