Question

EDIT: after reading all the input from the other users, i decided, to use what @chris suggested call_user_func_array() one more reason not to use eval() its slower than call_user_func_array(), but so far, nobody was able to exploit it my way, if you find a way, please post it as answer or comment :). So everybody can learn from it.
Merry XMAS to all!

—EDIT END—

Ok i needed to make a dynamic code:

I get user input like $_POST[‘a’], $_POST[‘b’]; // Depends on each query how many user input.

$sql = "SELECT 1, 2, 3 FROM x WHERE b = ? AND a = ? LIMIT 10"; // SQL STATEMENT
$input = array($_POST['a'], $_POST['b']);
$output = 3; // Number of variables need for 1, 2, 3
$data = readDB2($sql, $input, $output);
var_dump($data);

this input, gets passed to mysqli->prepared statements

cause the number of variables is dynamic ($input and $output);

i used the php function eval(); Now my question can this be exploited, in my code?

Just look in my function readDB2 to see how i used the eval() function (used it 3x times).

public function readDB2($sql, $input, $output1) {

    $stmt = $this->mysqli->prepare($sql);
    if(!empty($input) && is_array($input)) {
        $sp = "";
        $data = "";
        $inputn = count($input) - 1;
        for($i = 0; $i <= $inputn; $i++) {
            if($i !== $inputn) {
                $data .= '$input[' . $i . "],";
            } else {
                $data .= '$input[' . $i . "]";
            }
            $sp .= "s";
        }
        $bind = '$stmt->bind_param(\''. $sp . '\',' . $data . ');';
        eval("return $bind");
    }
    if (!$stmt) {throw new Exception($this->mysqli->error);}
    $stmt->execute();
    if (!$stmt) {throw new Exception($this->mysqli->error);}
    $stmt->store_result();
    $checker = $stmt->num_rows;
    if($checker !== 0) {
        if(!empty($output1)) {
            $out = "";
            for($i = 1; $i <= $output1; $i++) {
                if($i !== $output1) {
                    $out .= '$out' . $i . ",";
                } else {
                    $out .= '$out' . $i;
                }
            }
            $res = '$stmt->bind_result(' . $out . ');';
            eval("return $res");

            $vars = "array(" . $out . ");";

            while ($stmt->fetch()) {
                $results[] = eval("return $vars");
            }

        }
    } else {
        $results = "NO RESULTS";
    }
    $stmt->fetch();
    $stmt->close();

    $this->results = array('num_rows' => $checker, $results);

    return $this->results;
}

EDIT FOR meagar

$bind = '$stmt->bind_param(\''. $sp . '\',' . $data . ');'; 
==
$bind = '$stmt->bind_param('ss', $input[0], $input[1]);); 
OR and so on
$bind = '$stmt->bind_param('sss', $input[0], $input[1], $input[2]););

EDIT FOR Incognito:

$input = array($_POST['pwnd']);

$data = readDB2($sql, $input, $output) {

public function readDB2($sql, $input, $output) {
    ...
    $inputn = count($input) - 1;
    for($i = 0; $i <= $inputn; $i++) {
            if($i !== $inputn) {
                $data .= '$input[' . $i . "],";
            } else {
                $data .= '$input[' . $i . "]";
            }
            $sp .= "s";
        }
        $bind = '$stmt->bind_param(\''. $sp . '\',' . $data . ');';
        eval("return $bind");

    ...
}

in my result

$bind = '$stmt->bind_param(\''. $sp . '\',' . $data . ');';

gets

eval("return $bind");

gets

$stmt->bind_param('s', $input[0]);

not what you said.

Answer 1

fyi, call_user_func_array() is how you call functions with unknown number of arguments.

array_unshift($input, str_repeat('s', count($input)));
$callable = array($stmt, 'bind_param');
call_user_func_array($callable, $input);

array_unshift() pushes the ‘sss’ string element to the front of the array(we want the front because it needs to be the first argument fed to bind_param)

$callable is the callback psuedo type

also, in the future, if you find yourself using eval, familiarize yourself with php’s var_export() function which can assist you with constructing safe strings. Try not to use eval though.