Handling and Error Handling

The first point is do you really need these assignments? can not use $_POST direct? If you can not the two best ways to test the existence of a key are.

1) isset() / empty() plus ternary usage

Basically, isset()/empty() checks for existence (according to its criteria) if $idade receives the value of the array of the counter it receives a default value, in case 99.

$arr = array('nome' => 'abc', 'email' => '[email protected]');
$idade = !isset($arr['idade']) ? $arr['idade'] : 99;

2) Use of the null coalescing operator ( ?? ) available only in PHP7.

It checks if the key exists if it does the assignment with its value or otherwise the default value.

$idade = $arr['idade'] ?? 99;

Something really practical and recommended is to make use of ternary operators along with the isset and empty functions as in the @rray response, to avoid reading indexes that do not exist.

Something I also like to use when I need variables with the same index name assigned in the form (or even because of large forms) , and at the same time avoid errors like this (undefined index) is iterating the variable $_POST by integer and deleting the values that I do not need.

# valores em $_POST, provenientes do formulário
Array ( [nome] => edilson [apelido] => samuel [email] => [enviar] => enviar )

if(isset($_POST)){
    foreach(array_keys($_POST) as $input)
    {
        switch($input):
            case 'enviar':
            case 'outro':
                unset($_POST[$input]);
                continue;
            default:
                ${$input} = $_POST[$input];
                print "${$input} : $input <br>";
                break;
        endswitch;  

    }
    if($email){
        print "<br>email definido<br>";
    } else {
        print "<br>email não definido<br>";
    }
}

In case of filters, it applies directly to ${$input} , which results in something like this:

${$input} = validar($_POST[$input]);