Catch part of a URL with PHP Explode

If you want to get the parameters passed by GET from any URL - not necessarily from the current page, which PHP already makes available in $_GET - there are functions that are suitable for this:

Putting the two together:

$url = "https://www.dominio.com/login?erro=1&data=2018-03-06";
$partes = parse_url($url);
if(!empty($partes['query'])) {
    $vars = [];
    parse_str($partes['query'], $vars);
    var_dump($vars);
}

// Saída:
//
// array(2) {
//   ["erro"]=>
//   string(1) "1"
//   ["data"]=>
//   string(10) "2018-03-06"
// }

See working on Ideone .

About the approach you were trying:

  

How do I get only what's after? and before &?

Thinking about this problem in a generic way (because to treat URLs I would use the above code), it is simple to solve with string manipulation. With explode you have already managed to get a string like this:

$sua_string = "erro=1&data=2018-03-06";

Looking at this, just find the first & and get everything that is before it:

$sua_string = "erro=1&data=2018-03-06";
$pos = strpos($sua_string, '&');
$resultado = substr($sua_string, 0, $pos-1);
echo $resultado; // erro=1

Everything that comes after ? is populated by global super $_GET , so if you want to get some parameter in the url you can do the following:

$data = $_GET['data'];

This code would return the value '2018-03-06' .

To get the error just do:

$erro = $_GET['erro'];

This code would return the value 1 .

As you want the value with the name of the variable, you can concatenate the value with a string.

$erroString = 'erro='.$erro;

The explode will transform $ url_cod into an array, to get what is before just put:

$parte_url = $url_cod[1];
//Ou [0] para oque vem antes

For the '&' you use the same system

Or if you want to get the GET parameters, do as @Phelipe answered above

 $url = "www.google.com?&nome_da_variavel_recebida=123";
 $valor = $_GET['nome_da_variavel_recebida'];
 //Nesse caso $valor seria igual a 123