Get value of variable by name in case it is array item, possibly from superglobal

Question

Is this at all possible to get a value from variable accessed by name in case it refers to an indexed array item? Actually I planned to do this trick with $_SERVER['HTTP_HOST'] but I found out that $$ doesn't get well with any specific array item.

$a = '555';
$b = 'a';
var_dump($$b);

$a = array (1,2,3);
$b = 'a';
var_dump($$b);
$b = 'a[1]';
var_dump($$b);

First two vardumps work as expected, but the last one gives Notice: Undefined variable: a[1] even though it obviously exists. And I want it to return 2.

upd: PHP 7.1.16

Answer 1

Here is simple parser for just arrays of strings or numbers

function parse($str) {
    // regex explanation:
    // ^ - beging of the string
    // valid name inside capture group with at least one character
    // \[ literal open bracket
    // ([^\]]+) - capture group with non closing bracket
    // \] - closing bracket
    // $ - end of string
    $re = "/^([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)\[([^\]]+)\]$/";
    if (preg_match($re, $str, $match)) {
        $var = $match[1];
        global $$var;
        // regex for any integer
        if (preg_match("/^[0-9]+$/", $match[2])) {
            // convert integera like string to integer
            return $$var[intval($match[2])];
        } else {
            // remove quotes from beginning and ending of a string
            return $$var[trim($match[2], "\"'")];
        }
    } else {
        return $$str;
    }
}

$a = array (1,2,3);
$b = 'a';
var_dump($$b);
$b = 'a[1]';
var_dump(parse($b));

$a = array("foo" => "hello");
$b = 'a["foo"]';
var_dump(parse($b));
echo parse("_SERVER['HTTP_HOST']");

regex for variable name can be found in php docs

the only limitation is that variable need to be global, but _SERVER is always global so it shouldn't matter.